libceph: fix overflow in osdmap_decode()

(cherry picked from commit e91a9b639a691e0982088b5954eaafb5a25c8f1c)

On 32-bit systems, a large `n' would overflow `n * sizeof(u32)' and bypass
the check ceph_decode_need(p, end, n * sizeof(u32), bad).  It would also
overflow the subsequent kmalloc() size, leading to out-of-bounds write.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Reviewed-by: Alex Elder <elder@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>