libceph: add process_one_ticket() helper

Add a helper for processing individual cephx auth tickets.  Needed for
the next commit, which deals with allocating ticket buffers.  (Most of
the diff here is whitespace - view with git diff -b).

Cc: stable@vger.kernel.org
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@redhat.com>