UPSTREAM: bpf: skmsg, replace comments with BUILD bug

Enforce comment on structure layout dependency with a BUILD_BUG_ON
to ensure the condition is maintained.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Change-Id: Ic13e65176618207ed60d6b378081b9415112d98d
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>