bcachefs: Fix __bch2_buffered_write() returning -ENOMEM

When grab_cache_page_write_begin() fails but we did pin some pages, we
shouldn't return -ENOMEM, we should do a partial write.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-io.c

@@ -1606,9 +1606,14 @@ static int __bch2_buffered_write(struct bch_inode_info *inode,
 		pages[i] = grab_cache_page_write_begin(mapping, index + i);
 		if (!pages[i]) {
 			nr_pages = i;
+			if (!i) {
 				ret = -ENOMEM;
 				goto out;
 			}
+			len = min_t(unsigned, len,
+				    nr_pages * PAGE_SIZE - offset);
+			break;
+		}
 	}
 
 	if (offset && !PageUptodate(pages[0])) {