proc: fix missing final NUL in get_mm_cmdline() rewrite

The rewrite of the cmdline fetching missed the fact that we used to also
return the final terminating NUL character of the last argument.  I
hadn't noticed, and none of the tools I tested cared, but something
obviously must care, because Michal Kubecek noticed the change in
behavior.

Tweak the "find the end" logic to actually include the NUL character,
and once past the eend of argv, always start the strnlen() at the
expected (original) argument end.

This whole "allow people to rewrite their arguments in place" is a nasty
hack and requires that odd slop handling at the end of the argv array,
but it's our traditional model, so we continue to support it.

Repored-and-bisected-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-and-tested-by: Michal Kubecek <mkubecek@suse.cz>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>