ext4: use iomap for zeroing blocks in DAX mode

Use iomap infrastructure for zeroing blocks when in DAX mode.
ext4_iomap_begin() handles read requests just fine and that's all that
is needed for iomap_zero_range().

Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>