libnetfilter: Avoid using VLAs

VLAs in composite data types like structures and unions
are not a standard feature of C language,  gcc has specific
implementations for but other compilers dont have that done specifically clang, and
the community refuses to implement it since its non standard.

Change-Id: I6ae24adb455bf262fe9406a1c8e3b3a4a0cf77d4
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>