xfstests: coalesce contiguous extents in extent map output

xfstests: coalesce contiguous extents in extent map output

The specific set of extent sizes allocated to a file is not always
deterministic.  In particular, sometimes a range of unwritten blocks
is covered by a single extent, while in other cases it might be
represented by multiple consecutive unwritten extents.  This can
result in spurious errors being reported in tests that check file
extent maps.

Add a filter that finds adjacent extents in what gets produced for
fiemap and bmap output and coalesces them as if all consective
extents of the same time were really just one extent.  (Note that
as implemented here this applies to all extent types, not just
unwritten extents.)

Update the golden output for test 242 to reflect the change.

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>