btrfs: make btrfs_split_bio work on struct btrfs_bio

btrfs: make btrfs_split_bio work on struct btrfs_bio

[ Upstream commit 2cef0c79bb81d8bae1dbc45195771a824ca45e76 ]

btrfs_split_bio expects a btrfs_bio as argument and always allocates one.
Type both the orig_bio argument and the return value as struct btrfs_bio
to improve type safety.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: c731cd0b6d25 ("btrfs: fix file_offset for REQ_BTRFS_ONE_ORDERED bios that get split")
Signed-off-by: Sasha Levin <sashal@kernel.org>