afs: Fix missing/incorrect unlocking of RCU read lock

In afs_proc_addr_prefs_show(), we need to unlock the RCU read lock in both
places before returning (and not lock it again).

Fixes: f94f70d3

 ("afs: Provide a way to configure address priorities")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202401172243.cd53d5f6-oliver.sang@intel.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org