nfs-utils: specify a create mode with open(...O_CREAT) call in xflock

The xflock function can create a file via open() with O_CREAT, but does
not specify the create mode when it does so. I think 0644 should be
appropriate given the current usage of this function.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Neil Brown <neilb@suse.de>