swap_pager: Unbusy readahead pages after an I/O error

The swap pager itself allocates readahead pages, so should take care to
unbusy them after a read error, just as it does in the non-error case.

PR:		277538
Reviewed by:	olce, dougm, alc, kib
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D44646

(cherry picked from commit 4696650782)

sys/vm/swap_pager.c

@@ -1686,6 +1686,9 @@ swp_pager_async_iodone(struct buf *bp)
 				 * getpages so don't play cute tricks here.
 				 */
 				vm_page_invalid(m);
+				if (i < bp->b_pgbefore ||
+				    i >= bp->b_npages - bp->b_pgafter)
+					vm_page_free_invalid(m);
 			} else {
 				/*
 				 * If a write error occurs, reactivate page