block, dax: move "select DAX" from BLOCK to FS_DAX

block, dax: move "select DAX" from BLOCK to FS_DAX

For configurations that do not enable DAX filesystems or drivers, do not
require the DAX core to be built.

Given that the 'direct_access' method has been removed from
'block_device_operations', we can also go ahead and remove the
block-related dax helper functions from fs/block_dev.c to
drivers/dax/super.c. This keeps dax details out of the block layer and
lets the DAX core be built as a module in the FS_DAX=n case.

Filesystems need to include dax.h to call bdev_dax_supported().

Cc: [email protected]
Cc: Jens Axboe <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: "Darrick J. Wong" <[email protected]>
Cc: Ross Zwisler <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Reported-by: Geert Uytterhoeven <[email protected]>
Signed-off-by: Dan Williams <[email protected]>