btrfs: handle errors from async submission

Btrfs' async submit mechanism is able to handle errors in the submission
path and the meta-data async submit function correctly passes the error
code to the caller.

In btrfs_submit_bio_start() and btrfs_submit_bio_start_direct_io() we're
not handling the errors returned by btrfs_csum_one_bio() correctly though
and simply call BUG_ON(). This is unnecessary as the caller of these two
functions - run_one_async_start - correctly checks for the return values
and sets the status of the async_submit_bio. The actual bio submission
will be handled later on by run_one_async_done only if
async_submit_bio::status is 0, so the data won't be written if we
encountered an error in the checksum process.

Simply return the error from btrfs_csum_one_bio() to the async submitters,
like it's done in btree_submit_bio_start().

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -2160,11 +2160,8 @@ static blk_status_t btrfs_submit_bio_start(void *private_data, struct bio *bio,
 				    u64 bio_offset)
 {
 	struct inode *inode = private_data;
-	blk_status_t ret = 0;
 
-	ret = btrfs_csum_one_bio(BTRFS_I(inode), bio, 0, 0);
-	BUG_ON(ret); /* -ENOMEM */
-	return 0;
+	return btrfs_csum_one_bio(BTRFS_I(inode), bio, 0, 0);
 }
 
 /*
@@ -7618,10 +7615,8 @@ static blk_status_t btrfs_submit_bio_start_direct_io(void *private_data,
 				    struct bio *bio, u64 offset)
 {
 	struct inode *inode = private_data;
-	blk_status_t ret;
-	ret = btrfs_csum_one_bio(BTRFS_I(inode), bio, offset, 1);
-	BUG_ON(ret); /* -ENOMEM */
-	return 0;
+
+	return btrfs_csum_one_bio(BTRFS_I(inode), bio, offset, 1);
 }
 
 static void btrfs_end_dio_bio(struct bio *bio)