ceph: allow encrypting a directory while not having Ax caps

If a client doesn't have Fx caps on a directory, it will get errors while
trying encrypt it:

ceph: handle_cap_grant: cap grant attempt to change fscrypt_auth on non-I_NEW inode (old len 0 new len 48)
fscrypt (ceph, inode 1099511627812): Error -105 getting encryption context

A simple way to reproduce this is to use two clients:

client1 # mkdir /mnt/mydir

client2 # ls /mnt/mydir

client1 # fscrypt encrypt /mnt/mydir
client1 # echo hello > /mnt/mydir/world

This happens because, in __ceph_setattr(), we only initialize
ci->fscrypt_auth if we have Ax and ceph_fill_inode() won't use the
fscrypt_auth received if the inode state isn't I_NEW. Fix it by allowing
ceph_fill_inode() to also set ci->fscrypt_auth if the inode doesn't have
it set already.

Signed-off-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>