Fix inverse sleep logic in buf_daemon().

Before commit 3cec5c77d6 buf_daemon() went to longer 1s sleep if
numdirtybuffers <= lodirtybuffers.  After that commit new condition
!BIT_EMPTY(BUF_DOMAINS, &bdlodirty) got opposite -- true when one
or more more domains is above lodirtybuffers.  As result, on freshly
booted system with no dirty buffers buf_daemon() wakes up 10 times
per second and probably only 1 time per second when there is actual
work to do.

MFC after:	1 week
Reviewed by:	kib, markj
Tested by:	pho
Differential revision:	https://reviews.freebsd.org/D33890

sys/kern/vfs_bio.c

@@ -3464,7 +3464,7 @@ buf_daemon()
 		 * to avoid endless loops on unlockable buffers.
 		 */
 		mtx_lock(&bdlock);
-		if (!BIT_EMPTY(BUF_DOMAINS, &bdlodirty)) {
+		if (BIT_EMPTY(BUF_DOMAINS, &bdlodirty)) {
 			/*
 			 * We reached our low water mark, reset the
 			 * request and sleep until we are needed again.