Btrfs: fix missing error return if writeback for extent buffer never started

If lock_extent_buffer_for_io() fails, it returns a negative value, but its
caller btree_write_cache_pages() ignores such error. This means that a
call to flush_write_bio(), from lock_extent_buffer_for_io(), might have
failed. We should make btree_write_cache_pages() notice such error values
and stop immediatelly, making sure filemap_fdatawrite_range() returns an
error to the transaction commit path. A failure from flush_write_bio()
should also result in the endio callback end_bio_extent_buffer_writepage()
being invoked, which sets the BTRFS_FS_*_ERR bits appropriately, so that
there's no risk a transaction or log commit doesn't catch a writeback
failure.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -3978,6 +3978,10 @@ int btree_write_cache_pages(struct address_space *mapping,
 			if (!ret) {
 				free_extent_buffer(eb);
 				continue;
+			} else if (ret < 0) {
+				done = 1;
+				free_extent_buffer(eb);
+				break;
 			}
 
 			ret = write_one_eb(eb, wbc, &epd);