init/initramfs: Fix argument forwarding to panic() in panic_show_mem()

Forwarding variadic argument lists can't be done by passing a va_list
to a function with signature foo(...) (as panic() has). It ends up
interpreting the va_list itself as a single argument instead of
iterating it. printf() happily accepts it of course, leading to corrupt
output.

Convert panic_show_mem() to a macro to allow forwarding the arguments.
The function is trivial enough that it's easier than trying to introduce
a vpanic() variant.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>