dax: guarantee page aligned results from bdev_direct_access()

dax: guarantee page aligned results from bdev_direct_access()

Orabug: 22913653

If a ->direct_access() implementation ever returns a map count less than
PAGE_SIZE, catch the error in bdev_direct_access().  This simplifies
error checking in upper layers.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reported-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit fe683adabfe6629c0b6db32bbbc1ce6cacbf2117)
Signed-off-by: Dan Duval <dan.duval@oracle.com>