afs: Fix updating of i_mode due to 3rd party change

Fix afs_apply_status() to mask off the irrelevant bits from status->mode
when OR'ing them into i_mode.  This can happen when a 3rd party chmod
occurs.

Also fix afs_inode_init_from_status() to mask off the mode bits when
initialising i_mode.

Fixes: 260a9803 ("[AFS]: Add "directory write" support.")
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>