ARC refcount fixes

[behlendorf]

Motivation and Context

Resolve #7219 which is occasionally encountered by ztest and was introduced by the encryption changes.

Additionally, resolve an unrelated reference count discrepancy accidentally introduced when porting compressed send/recv. This may potentially explain #7820 though I was unable to reproduce the symptoms locally.

Description

• Fix arc_relase() refcount

  Update arc_release to use arc_buf_size(). This hunk was accidentally
  dropped when porting compressed send/recv, 2aa3438.

• Add zfs_refcount_transfer_ownership_many()

  When debugging is enabled and a zfs_refcount_t contains multiple holders
  using the same key, but different ref_counts, the wrong reference_t may
  be transferred. Add a zfs_refcount_transfer_ownership_many() function,
  like the existing zfs_refcount_*_many() functions, to match and transfer
  the correct refcount_t;

  This issue may occur when using encryption with refcount debugging
  enabled. An arc_buf_hdr_t can have references for both the
  hdr->b_l1hdr.b_pabd and hdr->b_crypt_hdr.b_rabd which both use the
  hdr as the reference holder. When unsharing the buffer the p_abd
  should be transferred.

  This issue does not impact production builds because refcount holders
  are not tracked.

How Has This Been Tested?

Local ztest runs. Prior to this change I was able to reproduce the #7219 approximately once an hour using ztest. With the fix applied I've thus far been unable to reproduce the issue. I'm pushing this change to the CI for additional ztest coverage and will let my local ztest run overnight.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.
• Change has been approved by a ZFS on Linux member.

[tcaputi]

LGTM. Maybe add a comment somewhere that notes that refcounts adds / removes must be matched.

[codecov]

Codecov Report

Merging #8000 into master will decrease coverage by 0.26%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #8000      +/-   ##
==========================================
- Coverage   78.68%   78.41%   -0.27%     
==========================================
  Files         377      377              
  Lines      114213   114215       +2     
==========================================
- Hits        89871    89566     -305     
- Misses      24342    24649     +307

Flag	Coverage Δ
#kernel	78.7% <42.85%> (-0.17%)	⬇️
#user	67.56% <71.42%> (-0.46%)	⬇️

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[behlendorf]

We should cherry-pick 5e8ff25 for 0.7.12.