This commit improves the section mismatch warning format when there is
no suitable symbol name to print.

The section mismatch warning prints the reference source in the form
of <symbol_name>+<offset> and the reference destination in the form
of <symbol_name>.

However, there are some corner cases where <symbol_name> becomes
"(unknown)", as reported in commit 23dfd914 ("modpost: fix null
pointer dereference").

In such cases, it is better to print the symbol address.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>