Check for bogus_page in vnode_pager_generic_getpages_done().

We now assert that a page is busy when updating its validity-tracking
state, but bogus_page is not busied during a getpages operation.

Reported by:	syzkaller
Reviewed by:	alc, kib
Discussed with:	jeff
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D22124

sys/vm/vnode_pager.c

@@ -1149,6 +1149,8 @@ vnode_pager_generic_getpages_done(struct buf *bp)
 
 		nextoff = tfoff + PAGE_SIZE;
 		mt = bp->b_pages[i];
+		if (mt == bogus_page)
+			continue;
 
 		if (nextoff <= object->un_pager.vnp.vnp_size) {
 			/*