'zfs share -a' should clean noauto exports

[don-brady]

Motivation and Context

This is a follow on to PR-10688 where zfs share -a allows the sharing of canmount=noauto datasets if they are mounted.
However, when a dataset with canmount=noauto is not mounted, the command should also purge any existing entries from the exports file. Otherwise, after a reboot, the nfs server attempts to export the underlying mountpath, not the dataset. This can lead to a hard hang for existing client mounts.

Reference: https://access.redhat.com/solutions/3540591

Description

Instead of just skipping the adding of an export if not mounted and canmount=noauto, have it also remove an existing export of the dataset so that, after a reboot, we don't export an unmounted dataset.

How Has This Been Tested?

ZTS - functional/cli_root/zfs_share/zfs_share_012_pos.ksh (new test to verify behavior)

Also tested in our internal testing workflows that was previously causing a hard hang

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.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov]

Codecov Report

Merging #10747 into master will decrease coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #10747      +/-   ##
==========================================
- Coverage   79.70%   79.54%   -0.16%     
==========================================
  Files         394      394              
  Lines      124638   124640       +2     
==========================================
- Hits        99342    99145     -197     
- Misses      25296    25495     +199     

Flag	Coverage Δ
#kernel	80.30% <ø> (-0.07%)	⬇️
#user	65.11% <100.00%> (-0.85%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/zfs/zfs_main.c	82.86% <100.00%> (+0.06%)	⬆️
module/zfs/vdev_rebuild.c	93.69% <0.00%> (-3.48%)	⬇️
module/zfs/vdev_raidz.c	88.88% <0.00%> (-2.62%)	⬇️
module/zfs/dsl_synctask.c	92.40% <0.00%> (-2.54%)	⬇️
lib/libzfs/libzfs_changelist.c	84.26% <0.00%> (-1.88%)	⬇️
module/zfs/lzjb.c	98.14% <0.00%> (-1.86%)	⬇️
cmd/zed/agents/fmd_api.c	88.61% <0.00%> (-1.78%)	⬇️
module/zfs/metaslab.c	94.35% <0.00%> (-1.49%)	⬇️
module/os/linux/zfs/vdev_disk.c	84.04% <0.00%> (-1.17%)	⬇️
cmd/zed/agents/zfs_diagnosis.c	77.55% <0.00%> (-1.17%)	⬇️
... and 46 more

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