Restructure nvlist_nv_alloc to work on FreeBSD

[mattmacy]

KM_PUSHPAGE is an Illumosism - On FreeBSD it's
aliased to the same malloc flag as KM_SLEEP.
The compiler naturally rejects multiple case
statements with the same value.

Signed-off-by: Matt Macy [email protected]

Motivation and Context

Description

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.

[lundman]

Making default be pushpage is probably ok, all places does seem to specify a flag and not just "0".

[codecov]

Codecov Report

Merging #9643 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #9643      +/-   ##
==========================================
- Coverage   79.47%   79.46%   -0.02%     
==========================================
  Files         418      418              
  Lines      123544   123544              
==========================================
- Hits        98183    98170      -13     
- Misses      25361    25374      +13

Flag	Coverage Δ
#kernel	80.02% <0%> (ø)	⬆️
#user	67.28% <ø> (+0.14%)	⬆️

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