bcachefs: Fix short buffered writes

bcachefs: Fix short buffered writes

In the buffered write path, we have to check for short writes that write
to the full page, where the page wasn't UpToDate; when this happens, the
page is partly garbage, so we have to zero it out and revert that part
of the write.

This check was wrong - we reverted total from copied, but didn't revert
the iov_iter, probably also leading to corrupted writes.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>