modpost: improve the section mismatch warning format

modpost: improve the section mismatch warning format

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 23dfd914d2bf ("modpost: fix null
pointer dereference").

In such cases, it is better to print the symbol address.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>