fault-inject: avoid call to random32() if fault injection is disabled

After enabling CONFIG_FAILSLAB I noticed random32 in profiles even if slub
fault injection wasn't enabled at runtime.

should_fail forces a comparison against random32() even if probability is
0:

        if (attr->probability <= random32() % 100)
                return false;

Add a check up front for probability == 0 and avoid all of the more
complicated checks.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>