LU-8191 selftest: restore BUILD_BUGs

LU-8191 selftest: restore BUILD_BUGs

It was wrong to remove lnet_selftest_structure_assertion()
since it contained BUILD_BUGs used to ensure different LNet
Selftest versions can interoperate.

Restore the BUILD_BUGs and add a dummy user in LNet Selftest
init. This should prevent analyzers from picking this up as
an unused function.

Fixes: 43cbc93 ("LU-8191 lnet: remove unused, fix non-static functions")
Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I5e498dc411048f0ae3ca69cc5d24a728a711d6f7