xfstests: move _filter_ln to common.filter

xfstests: move _filter_ln to common.filter

Move test 103's _filter_ln to common.filter and make
it more generic (not depending on 103's pathnames).

TBH I've lost my children's treasury of ln failure
messages, so I'm not sure this catches all variants;
it's hard to work backwards from the existing sed script
to what the various outputs were.  This works for me
but might need more tweaking on other systems.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>