xfstests: aio-stress is calling pthread_join incorrectly

xfstests: aio-stress is calling pthread_join incorrectly

The retval parameter to pthread_join() is a pointer to a pointer.
aio-stress is passing in a pointer to an int. This can result in a bus
error on sparc64 when the pointer is not aligned on a 64-bit boundary.
Since the return value is not used, it is simplest to pass a NULL
pointer instead.

This has been fixed in the LTP source:
https://github.com/linux-test-project/ltp/commit/981d33aad3b33c4625b88990fbf2fad5470d47e0

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>