Add missing zfs_exit() when snapdir is disabled (#16912)

zfs_vget doesn't zfs_exit when erroring out due to snapdir
being disabled.

Signed-off-by: Andrew Walker <awalker@ixsystems.com>
Reviewed-by: @bmeagherix
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>