proc: don't use READ_ONCE/WRITE_ONCE for /proc/*/fail-nth

READ_ONCE and WRITE_ONCE are useless when there is only one read/write
is being made.

Link: http://lkml.kernel.org/r/20171120204033.GA9446@avx2


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>