GFS2: fallocate: do not rely on file_update_time to mark the inode dirty

Previously __gfs2_fallocate() relied on file_update_time() marking the
inode dirty, but that's not a safe assumption as that function doesn't
dirty the inode in some cases. Mark the inode dirty explicitly.

Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>