dlmfs_file_write(): fix the bogosity in handling non-zero *ppos

commit 3815f1be

 upstream.

'count' is how much you want written, not the final position.
Moreover, it can legitimately be less than the current position...

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>