fallocate + read/write tests, ext4 regression tests

fallocate + read/write tests, ext4 regression tests

New test to test basic mixed fallocate + read & write,
includes a couple regression tests for bugs that ext4
hit.  Uses xfs_io to generate fallocate calls, so requires
git xfsprogs and very recent glibc at this point.

Ext4 folks, this is hopefully a reasonable example of
how to add a new test.   :)

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>