FreeBSD: restore zfs_znode_update_vfs()

I accidentally removed this in c22d56e3e, and didn't notice because it
doesn't fail the build, but does fail to load into the kernel because it
can't link it.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16554