Linux: Fix zfs_prune panics v2

[snajpa]

Motivation and Context

The approach I've taken in #16770 was insufficient per @youzhongyang report here

Description

It turns out that approach taken in the original version of the patch was wrong. So now, we're taking approach in-line with how kernel actually does it - when sb is being torn down, access to it is serialized via sb->s_umount rwsem, only when that lock is taken is it okay to work with s_flags - and the other mistake I was doing was trying to make SB_ACTIVE work, but apparently the kernel checks the negative variant - not SB_DYING and not SB_BORN.

This way it finally seems to work without issues.

How Has This Been Tested?

Confirmed by @youzhongyang

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.