checkpatch: complain about ffs(3) calls

The ffs(3) family of functions is not portable.  MinGW doesn't always
provide the function.

Use ctz32() or ctz64() instead.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1427124571-28598-10-git-send-email-stefanha@redhat.com
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
