proc: deduplicate /proc/*/cmdline implementation

Code can be sonsolidated if a dummy region of 0 length is used in normal
case of \0-separated command line:

1) [arg_start, arg_end) + [dummy len=0]
2) [arg_start, arg_end) + [env_start, env_end)

Link: http://lkml.kernel.org/r/20180221193335.GB28678@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>