Get rid of __ prefix for internal frand state

It made (some) sense when we had both OS and frand in anon unions,
but now lets just kill it.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/trim.c b/trim.c
index a15263d..95c433b 100644
--- a/trim.c
+++ b/trim.c
@@ -75,7 +75,7 @@
 	if (!td->o.trim_percentage)
 		return 0;
 
-	r = __rand(&td->__trim_state);
+	r = __rand(&td->trim_state);
 	val = (FRAND_MAX / 100ULL);
 
 	val *= (unsigned long long) td->o.trim_percentage;