[truenas/zfs-2.1-release] Prevent panic during concurrent snapshot rollback and zvol read

[ixhamza]

Motivation and Context

Protect zvol_cdev_read with zv_suspend_lock to prevent concurrent release of the dnode, avoiding panic when a snapshot is rolled back in parallel during ongoing zvol read operation.
Closes: openzfs#14839

Description

In FreeBSD, when a zvol read operation is in progress, a snapshot rollback can lead to a panic if the volmode is set to dev. To prevent the panic, we protect the zvol_cdev_read function with the zv_suspend_lock. This ensures that the dnode is not concurrently released, thereby avoiding the panic.

How Has This Been Tested?

Tested through following commands in FreeBSD with zfs volmode set to dev.

truncate -s 8G /f1
zpool create -f tank /f1
zfs create -V 4G tank/zv1
zfs snapshot tank/zv1@sp1
dd if=/dev/urandom of=/dev/zvol/tank/zv1 bs=512 count=1M status=progress
zfs rollback -r tank/zv1@sp1 & dd if=/dev/zvol/tank/zv1 of=temp2 bs=512 count=1M status=progress

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.