ext4: cleanup the dax handling in ext4_fill_super

Only call fs_dax_get_by_bdev once the sbi has been allocated and remove
the need for the dax_dev local variable.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20211129102203.2243509-21-hch@lst.de


Signed-off-by: Dan Williams <dan.j.williams@intel.com>