ext4: use ext4_get_block_write() for DAX

ext4: use ext4_get_block_write() for DAX

Orabug: 22913653

DAX relies on the get_block function either zeroing newly allocated
blocks before they're findable by subsequent calls to get_block, or
marking newly allocated blocks as unwritten.  ext4_get_block() cannot
create unwritten extents, but ext4_get_block_write() can.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Reported-by: Andy Rudoff <andy.rudoff@intel.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit e676a4c191653787c3fe851fe3b9f1f33d49dac2)
Signed-off-by: Dan Duval <dan.duval@oracle.com>