Linux: fix 2 lowmem bugs

[snajpa]

Motivation and Context

Linux: Fix zfs_prune panics:
#16324

Linux: s_op: use .free_inode:
#16608

Description

Linux: Fix zfs_prune panics

by protecting against sb->s_shrink eviction on umount with newer kernels

deactivate_locked_super calls shrinker_free and only then
sops->kill_sb cb, resulting in UAF on umount when trying
to reach for the shrinker functions in zpl_prune_sb of
in-umount dataset

Linux: s_op: use .free_inode

as per Documentation/filesystems/porting.rst:

quote:

** strongly recommended **

take the RCU-delayed parts of ->destroy_inode() into a new method -
->free_inode().  If ->destroy_inode() becomes empty - all the better,
just get rid of it.

endquote.

How Has This Been Tested?

Low memory scenario docker pull with zfs as storage backend, now passes. Template build at vpsFree, also passes.

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)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS 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.

[satmandu]

@behlendorf Any chance of getting this reviewed for #16760 2.3.0-rc4?
(I'm biased as someone who makes heavy use of docker.)