btrfs: fix _filter_mkfs regression

btrfs: fix _filter_mkfs regression

5e8b9e6 btrfs: add regression test for remount with thread_pool resized

did weird things to _filter_mkfs; aside from broken indentation,
it also short-circuited the default non-xfs behavior, which was to
emit a default block & inode size.  And that was all because btrfs/082
was using _filter_mkfs & not redirecting output away as per normal.

Granted, it's not super clear that _filter_mkfs serves this rather
unique purpose, but anyway...

And, while having this default seems to be of questionable value,
not emitting *anything* led to this on btrfs:

    +./tests/generic/204: line 76: space / (isize + dbsize): division by 0 (error token is ")")

because those variables don't get set for btrfs, thanks to the
above commit.

So take out the use of _filter_mkfs in btrfs/082, and take out the
munging of _filter_mkfs which broke generic/204, and get things back
to something semi-sane.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>