xfsqa: Add fiemap exerciser

xfsqa: Add fiemap exerciser

Preliminary fiemap testing support based on a test util written by
Josef Bacik.

For now it's only run with preallocation disabled, because xfs has a
tendency to fill in holes with data blocks (EOF prealloc stuff I
think) and similar for explicit preallocation, so this is breaking
the preallocation tests for now, when it finds a "data" block where
it expects a preallocated block.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>