fstests: enable sparse checking with make C=[12]

fstests: enable sparse checking with make C=[12]

Enable "make C=1" sparse checking when files get rebuilt. To check
all files, run "make clean" first.

Enable "make C=2" sparse checking of all files without rebuilding them.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>