ext[234]: fix comment for nonexistent variable

The comment in ext[234]_new_blocks() describes about "i".  But there is no
local variable called "i" in that scope.  I guess it has been renamed to
group_no.

Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>