dax: move the dax_read_lock() locking into dax_supported

Move the dax_read_lock/dax_read_unlock pair from the callers into
dax_supported to make it a little easier to use.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/20210826135510.6293-6-hch@lst.de


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