fs/proc/base.c: save decrement during lookup/readdir in /proc/$PID

Comparison for "<" works equally well as comparison for "<=" but one
SUB/LEA is saved (no, it is not optimised away, at least here).

Link: http://lkml.kernel.org/r/20161122195143.GA29812@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>