fuse: add file_modified() to fallocate

Add missing file_modified() call to fuse_file_fallocate().  Without this
fallocate on fuse failed to clear privileges.

Fixes: 05ba1f08 ("fuse: add FALLOCATE operation")
Cc: <stable@vger.kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>