dax: move the partition alignment check into fs_dax_get_by_bdev

dax: move the partition alignment check into fs_dax_get_by_bdev

fs_dax_get_by_bdev is the primary interface to find a dax device for a
block device, so move the partition alignment check there instead of
wiring it up through ->dax_supported.

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/20211129102203.2243509-7-hch@lst.de
Signed-off-by: Dan Williams <dan.j.williams@intel.com>