ext4: check allocation failure when duplicating "data" in ext4_remount()

There is no check for allocation failure when duplicating
"data" in ext4_remount(). Check for failure and return
error -ENOMEM in this case.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>