generic: test race between block map change and writeback

generic: test race between block map change and writeback

Run delalloc writes & append writes & non-data-integrity syncs
concurrently to test the race between block map change vs writeback.

This is to cover an XFS bug that data could be written to wrong
block and delay allocated blocks are leaked because the block map
was changed due to the removal of speculative allocated eofblocks
when writeback is in progress.

And this test partially mimics what lustre-racer[1] test does, using
which this bug was first found.

[1] https://git.hpdd.intel.com/?p=fs/lustre-release.git;a=tree;f=lustre/tests/racer;hb=HEAD

Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>