proc: simpler iterations for /proc/*/cmdline

"rv" variable is used both as a counter of bytes transferred and an
error value holder but it can be reduced solely to error values if
original start of userspace buffer is stashed and used at the very end.

[akpm@linux-foundation.org: simplify cleanup code]
Link: http://lkml.kernel.org/r/20180221193009.GA28678@avx2


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>