osdep: Retry SETLK upon EINTR

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: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>