fuse: fix retrieve length

commit c9e67d483776d8d2a5f3f70491161b205930ffe1 upstream.

In some cases fuse_retrieve() would return a short byte count if offset was
non-zero.  The data returned was correct, though.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>