ext4: avoid duplicate writes of the backup bg descriptor blocks

The resize code was needlessly writing the backup block group
descriptor blocks multiple times (once per block group) during an
online resize.

Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org