modpost: pass 'tosec' down to default_mismatch_handler()

default_mismatch_handler() does not need to compute 'tosec' because
it is calculated by the caller.

Pass it down to default_mismatch_handler() instead of calling
sec_name() twice.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>