Btrfs: work around maybe-uninitialized warning

A rewrite of btrfs_submit_direct_hook appears to have introduced a warning:

fs/btrfs/inode.c: In function 'btrfs_submit_direct_hook':
fs/btrfs/inode.c:8467:14: error: 'bio' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Where the 'bio' variable was previously initialized unconditionally, it
is now set in the "while (submit_len > 0)" loop that would never execute
if submit_len is zero.

Assuming this cannot happen in practice, we can avoid the warning
by simply replacing the while{} loop with a do{}while() loop so
the compiler knows that it will always be entered at least once.

Fixes changes introduced in "Btrfs: use bio_clone_bioset_partial to
simplify DIO submit".
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -8507,7 +8507,7 @@ static int btrfs_submit_direct_hook(struct btrfs_dio_private *dip,
 	/* bio split */
 	ASSERT(map_length <= INT_MAX);
 	atomic_inc(&dip->pending_bios);
-	while (submit_len > 0) {
+	do {
 		clone_len = min_t(int, submit_len, map_length);
 
 		/*
@@ -8550,7 +8550,7 @@ static int btrfs_submit_direct_hook(struct btrfs_dio_private *dip,
 				      start_sector << 9, &map_length, NULL, 0);
 		if (ret)
 			goto out_err;
-	}
+	} while (submit_len > 0);
 
 submit:
 	ret = __btrfs_submit_dio_bio(bio, inode, file_offset, skip_sum,