ext2: Avoid DAX zeroing to corrupt data

Currently ext2 zeroes any data blocks allocated for DAX inode however it
still returns them as BH_New. Thus DAX code zeroes them again in
dax_insert_mapping() which can possibly overwrite the data that has been
already stored to those blocks by a racing dax_io(). Avoid marking
pre-zeroed buffers as new.

Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>