bcachefs: Fix swallowing of data in buffered write path

bcachefs: Fix swallowing of data in buffered write path

In __bch2_buffered_write, if we fail to write to an entire !uptodate
folio, we have to back out the write, bail out and retry.

But we were missing an iov_iter_revert() call, so the data written to
the folio was lost and the rest of the write shifted to the wrong
offset.

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