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: NDan Williams <dan.j.williams@intel.com>
Reported-by: NRoss Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>