LU-12625 build: reliable detection of struct timespec64

LU-12625 build: reliable detection of struct timespec64

existing configure check define struct inode on stack and this
may cause the following error with gcc8:
build/conftest.c: In function main
build/conftest.c:226:1: error: the frame size of 1032 bytes is
	larger than 1024 bytes [-Werror=frame-larger-than=]
which result in false result of the ckeck and then osd-ldiskfs
doesn't build.

put struct inode * on the stack instead.

Change-Id: If31cfd13836e36ef59d428d3c05bf7f51319f89b
Signed-off-by: Alexey Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35675
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>