libceph: safely decode max_osd value in osdmap_decode()

max_osd value is not covered by any ceph_decode_need().  Use a safe
version of ceph_decode_* macro to decode it.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>