Btrfs: fix BUG_ON in btrfs_submit_compressed_write

This is similar to btrfs_submit_compressed_read(), if we fail after
bio is allocated, then we can use bio_endio() and errors are saved
 in bio->bi_error.  But please note that we don't return errors to
its caller because the caller assumes it won't call endio to cleanup
on error.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/compression.c

@@ -402,7 +402,10 @@ int btrfs_submit_compressed_write(struct inode *inode, u64 start,
 			}
 
 			ret = btrfs_map_bio(root, WRITE, bio, 0, 1);
-			BUG_ON(ret); /* -ENOMEM */
+			if (ret) {
+				bio->bi_error = ret;
+				bio_endio(bio);
+			}
 
 			bio_put(bio);
 
@@ -432,7 +435,10 @@ int btrfs_submit_compressed_write(struct inode *inode, u64 start,
 	}
 
 	ret = btrfs_map_bio(root, WRITE, bio, 0, 1);
-	BUG_ON(ret); /* -ENOMEM */
+	if (ret) {
+		bio->bi_error = ret;
+		bio_endio(bio);
+	}
 
 	bio_put(bio);
 	return 0;