LU-11597 test: Fix sanityn 16a failed on arm

LU-11597 test: Fix sanityn 16a failed on arm

As now O_DIRECT expect IO aligned with PAGE SIZE,
x86_64 expect 4K size, but some other platform, it
could be 64K, use PAGE_SIZE here to make the test happy.

And macro O_DIRECT is defined if macro _GNU_SOURCE is defined
according to open man doc[1] and _GNU_SOURCE is defined at the
head of file fsx.c already. So set the value of OP_DIRECT to
O_DIRECT instead of hardcoding its value as O_DIRECT could have
different values for other platforms like Arm64[2].

[1]
https://man7.org/linux/man-pages/man2/open.2.html
"The O_DIRECT, O_NOATIME, O_PATH, and O_TMPFILE flags are Linux-
 specific.  One must define _GNU_SOURCE to obtain their definitions."
[2]
https://code.woboq.org/userspace/glibc/sysdeps/unix/sysv/linux/aarch64/bits/fcntl.h.html#_M/__O_DIRECT

Test-Parameters: testlist=sanityn envdefinitions=ONLY=16a
Fixes: 853d180121a6 ("LU-3606 fsx: Add fallocate operation to fsx")
Change-Id: If72d434adaf91a960dfc50c557d8b50793fda575
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
Reviewed-on: https://review.whamcloud.com/37589
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>