ext4: add empty lines to mkfs_filter

ext4: add empty lines to mkfs_filter

Mkfs program can dump empty lines if encounters warnings and results
in test to fail. One of the case when this happens is when we are
trying to create a big file system (> 4T) and if "big" is not present
in /etc/mke2fs.conf. Ignore those empty lines too.

Before the fix, test generic/472 failed with following diff

diff /root/xfstests/tests/generic/472.out /root/xfstests/results//generic/472.out.bad
@@ -1,6 +1,6 @@
 QA output created by 472
 +
 +
  regular swap
  too long swap
  tiny swap
After the fix, test generic/472 passed
Ran generic/472
Passed all 1 tests

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>