proc: selftests: shotgun testing of read/readdir/readlink/write

Perform reads with nearly everything in /proc, and some writing as well.

Hopefully memleak checkers and KASAN will find something.

[adobriyan@gmail.com: /proc/kmsg can and will block if read under root]
  Link: http://lkml.kernel.org/r/20180316232147.GA20146@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
[adobriyan@gmail.com: /proc/sysrq-trigger lives on the ground floor]
  Link: http://lkml.kernel.org/r/20180317164911.GA3445@avx2
Link: http://lkml.kernel.org/r/20180315201251.GA12396@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>