Btrfs: skip checksum verification if IO error occurs

Currently dio read also goes to verify checksum if -EIO has been returned,
although it usually fails on checksum, it's not necessary at all, we could
directly check if there is another copy to read.

And with this, the behavior of dio read is now consistent with that of
buffered read.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ use bool for uptodate ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -8180,6 +8180,7 @@ static int __btrfs_subio_endio_read(struct inode *inode,
 	int nr_sectors;
 	unsigned int pgoff;
 	int csum_pos;
+	bool uptodate = (err == 0);
 	int ret;
 
 	fs_info = BTRFS_I(inode)->root->fs_info;
@@ -8195,12 +8196,13 @@ static int __btrfs_subio_endio_read(struct inode *inode,
 
 		pgoff = bvec.bv_offset;
 next_block:
+		if (uptodate) {
 			csum_pos = BTRFS_BYTES_TO_BLKS(fs_info, offset);
 			ret = __readpage_endio_check(inode, io_bio, csum_pos,
-					bvec.bv_page, pgoff, start,
-					sectorsize);
+					bvec.bv_page, pgoff, start, sectorsize);
 			if (likely(!ret))
 				goto next;
+		}
 try_again:
 		done.uptodate = 0;
 		done.start = start;