proc: deduplicate /proc/*/cmdline implementation

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 <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>