Reserve DMU_BACKUP_FEATURE for ZSTD

[allanjude]

Reserve bit 25 in the zfs replication DMU_BACKUP_FEATURE bitfield for the ZSTD compression feature from FreeBSD.

Requires-builders: none

Description / Motivation

Reserve a bit in the DMU_BACKUP_FEATURE namespace for the ZSTD compression feature, to prevent future merge conflicts.

How Has This Been Tested?

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.

[behlendorf]

@allanjude would you mind adding your signed-of-by and force updating this PR so we can merge it.

[codecov]

Codecov Report

Merging #7626 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7626      +/-   ##
==========================================
+ Coverage   77.51%   77.56%   +0.04%     
==========================================
  Files         363      363              
  Lines      110517   110517              
==========================================
+ Hits        85672    85723      +51     
+ Misses      24845    24794      -51

Flag	Coverage Δ
#kernel	78.22% <ø> (-0.19%)	⬇️
#user	66.47% <ø> (+0.19%)	⬆️

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