LU-13274 uapi: make lustre UAPI headers C99 compliant

LU-13274 uapi: make lustre UAPI headers C99 compliant

Attempting to compile strict C99 user land applications or
libraries with the Lustre UAPI headers will fail. These same
errors can be seen by enabling CONFIG_UAPI_HEADER_TEST as well.
Update the Lustre UAPI headers to be compilable with -std=c99.
Enhance our current test covering UAPI header handling.

For OpenSFS branch we can't include <linux/stat.h> since we support
kernels before struct statx existed and they will collide with the
special definitions in lustre_user.h.

Change-Id: Ifb0da33180dc3c7e116d6bf2b7f603ad0528277a
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/37678
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Gian-Carlo DeFazio <defazio1@llnl.gov>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>