ext4: don't use bdev->bd_super in __ext4_journal_get_write_access

__ext4_journal_get_write_access already has a super_block available,
and there is no need to go from that to the bdev to go back to the
owning super_block.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Message-Id: <20230807112625.652089-3-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>