We could hit lock failure if there is a signal that makes fcntl return
-1 and errno set to EINTR. In this case we should retry.

Cc: qemu-stable@nongnu.org
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>