common: filter warning messages from mkfs.ext4

common: filter warning messages from mkfs.ext4

Commit 4a5cda8102828 ("xfstests: do not unmount tmpfs during remount")
changed generic/135 to no longer redirect stderr to /dev/null when
running _scratch_mkfs.  This caused ext4 using a 1k block size to fail
when running this test because mkfs.ext4 would issue a warning message
about the use of a non-block size that wasn't expected by golden
output for the test.  Fix this by filtering out warning messages in
_scratch_mkfs_ext4.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>