printk: Add a short description string to kmsg_dump()

kmsg_dump doesn't forward the panic reason string to the kmsg_dumper
callback.
This patch adds a new struct kmsg_dump_detail, that will hold the
reason and description, and pass it to the dump() callback.

To avoid updating all kmsg_dump() call, it adds a kmsg_dump_desc()
function and a macro for backward compatibility.

I've written this for drm_panic, but it can be useful for other
kmsg_dumper.
It allows to see the panic reason, like "sysrq triggered crash"
or "VFS: Unable to mount root fs on xxxx" on the drm panic screen.

v2:
 * Use a struct kmsg_dump_detail to hold the reason and description
   pointer, for more flexibility if we want to add other parameters.
   (Kees Cook)
 * Fix powerpc/nvram_64 build, as I didn't update the forward
   declaration of oops_to_nvram()

Signed-off-by: Jocelyn Falempe <[email protected]>
Acked-by: Petr Mladek <[email protected]>
Acked-by: Michael Ellerman <[email protected]> (powerpc)
Acked-by: Kees Cook <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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