mdadm: fix CFLAGS and ptest issues

mdadm: fix CFLAGS and ptest issues

* Pass global CFLAGS to build:

The CFLAGS does not pass to build at all since it was redefined by
mdadm Makefile:

  CFLAGS = $(CWFLAGS) $(CXFLAGS) ...

This could be done by setting 'CXFLAGS="${CFLAGS}"'.

* Also fix ptest build errors caused by global CFLAGS:

  raid6check.c:352:2: error: ignoring return value of posix_memalign, \
  declared with attribute warn_unused_result [-Werror=unused-result]

  raid6check.c:315:8: error: 'stripe_buf' may be used uninitialized \
  in this function [-Werror=maybe-uninitialized]

Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>