gfs2: properly initial file_lock used for unlock.

Rather than assuming all-zeros is sufficient, use the available API to
initialize the file_lock structure use for unlock.  VFS-level changes
will soon make it important that the list_heads in file_lock are
always properly initialized.

Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>