xfstests: handle xfs_quota output w/ long devicenames

xfstests: handle xfs_quota output w/ long devicenames

Long device names may be split onto their own line
on quota output:

Filesystem   Blocks  Quota  Limit Warn/Time    Mounted on
/dev/mapper/my-very-very-very-long-devicename
                48M      0      0  00 [------] /mnt/scratch

which breaks tests that capture quota output - currently,
only xfs/108.

Add a _filter_quota() which fixes this.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>