OpenZFS 8115 - parallel zfs mount

[don-brady]

Motivation and Context

Add support for parallel zfs mounts

OpenZFS-issue: https://www.illumos.org/issues/8115
OpenZFS-commit: openzfs/openzfs@a3f0e2b569

Description

Porting Notes:

• Use thread pools (tpool) API instead of introducing taskq interfaces to libzfs.
• Use pthread_mutext for locks as mutex_t isn't available.
• Ignore alternative libshare initialization since OpenZFS-7955 is not present on zfsonlinux.

How Has This Been Tested?

1. ZTS for cli_root
2. Manual testing with 2000 zfs file systems (2 levels) to confirm that both zpool import and zfs mount -a have improved mount times due to parallelization.

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.

[codecov]

Codecov Report

Merging #8092 into master will increase coverage by <.01%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master    #8092      +/-   ##
==========================================
+ Coverage   78.46%   78.46%   +<.01%     
==========================================
  Files         377      377              
  Lines      114518   114612      +94     
==========================================
+ Hits        89851    89930      +79     
- Misses      24667    24682      +15

Flag	Coverage Δ
#kernel	78.72% <ø> (-0.05%)	⬇️
#user	67.43% <96.29%> (ø)	⬆️

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[don-brady]

Looks like there is an unexpected interaction between libzfs_mnttab_add and libzfs_mnttab_update that is breaking some of the cli_root/zfs_unmount tests. Investigating.