libceph: eliminate unnecessary allocation in process_one_ticket()

Commit c27a3e4d ("libceph: do not hard code max auth ticket len")
while fixing a buffer overlow tried to keep the same as much of the
surrounding code as possible and introduced an unnecessary kmalloc() in
the unencrypted ticket path.  It is likely to fail on huge tickets, so
get rid of it.

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>