loop: Update mtime when writing using aops

Update mtime when writing to backing filesystem using the address space
operations write_begin and write_end.

Signed-off-by: Nikanth Karthikesan <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/loop.c

···
 		if (ret)
 			goto fail;
 
+		file_update_time(file);
+
 		transfer_result = lo_do_transfer(lo, WRITE, page, offset,
 				bvec->bv_page, bv_offs, size, IV);
 		copied = size;