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>

fs/bcachefs/fs-io-buffered.c

@@ -909,6 +909,7 @@ static int __bch2_buffered_write(struct bch_inode_info *inode,
 		if (!folio_test_uptodate(f) &&
 		    f_copied != folio_size(f) &&
 		    pos + copied + f_copied < inode->v.i_size) {
+			iov_iter_revert(iter, f_copied);
 			folio_zero_range(f, 0, folio_size(f));
 			folios_trunc(&folios, fi);
 			break;