ceph: factor out ceph_lookup_inode()

This function will be used by __fh_to_dentry and by the quotas code, to
find quota realm inodes that are not visible in the mountpoint.

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>