ceph: fix endianness bug when handling MDS session feature bits

Eduard reported a problem mounting cephfs on s390 arch. The feature
mask sent by the MDS is little-endian, so we need to convert it
before storing and testing against it.

Cc: stable@vger.kernel.org
Reported-and-Tested-by: Eduard Shishkin <edward6@linux.ibm.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>