NFS access to snapshots does not work for FreeBSD due to mnt_exjail not being filled in

[rmacklem]

System information

Type	Version/Name
Distribution Name	FreeBSD
Distribution Version	FreeBSD 14.0
Kernel Version	14.0-CURRENT
Architecture	amd64
OpenZFS Version	zfs-kmod-2.2.0-FreeBSD_g009d3288

When mounted over NFS, snapshots in .zfs/snapshot
are not accessible.

Export the dataset and mount it from a client and then
attempt to "cd /dataset/.zfs/snapshot/snapshot-name".

This is caused by the mnt_exjail field of the snapshot's
automounted vfs_t being NULL. After FreeBSD commit
88175af, this field must be filled in for NFS access to be
allowed.

The patch at https://reviews.freebsd.org/D42672 seems
to fix the problem.

[KungFuJesus]

Should be closed with: #15563
Though, I imagine this should be backported to 2.2.x?

[AllKind]

Then best to make a request here? #15602