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

Signed-off-by: Pavel Snajdr <[email protected]>

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.

Signed-off-by: Pavel Snajdr <[email protected]>