xfstests generic/260: get correct trimmed bytes

xfstests generic/260: get correct trimmed bytes

Starting from util-linux v2.23 fstrim(1) reports trimmed bytes
differently, e.g.

new fstrim: /mnt/ext4: 9.7 GiB (10411118592 bytes) trimmed
old fstrim: /mnt/ext4: 10411118592 bytes were trimmed

generic/260 reports syntax error

    +./tests/generic/260: line 111: [: 9.7: integer expression expected
    +./tests/generic/260: line 121: [: 9.7: integer expression expected
    +./tests/generic/260: line 183: [: 9.7: integer expression expected

Add a new filter called _filter_fstrim in common/filter and get the
correct trimmed bytes in generic/260, so the test passes with both old
and new fstrim.

Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>