dax: Fix compile with !CONFIG_DAX

Thanks to Vishal for reporting this one.
prd: Improve wording in Kconfig

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
prd: Updates to comments & whitespace

Update some comments and fix whitespace for checkpatch.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
prd: add support for rw_page()

Based on commit a72132c31d58 brd: add support for rw_page()

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
prd: remove redundant checks in prd_direct_access

Checks for sector alignment and capacity are now done in common code in
bdev_direct_access().

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
prd: Remove Kconfig default for PRD

The implicit default is "n", so in most cases PRD won't be built.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>