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cache: clear block address on uncache

if we leave the block address in the LRU, we may
end up uncaching an unrelated block when we pluck
the block off the lru:

	uncache(A[addr=x])
	freeblk(A[addr=x])
	cache(newblk()) => B(addr=x)
	pluck() // unache(A[addr=x]); clobbers B

this is bad, so lets not do it.

--- a/cache.c

+++ b/cache.c

@@ -67,6 +67,8 @@

 		fs->ctail->cnext = b;

 	if(fs->chead == nil)

 		fs->chead = b;

+	b->bp.addr = -1;

+	b->bp.hash = -1;

 	b->cprev = fs->ctail;

 	fs->ctail = b;

 	rwakeup(&fs->lrurz);

@@ -110,6 +112,8 @@

 	for(b = bkt->b; b != nil; b = b->hnext){

 		if(b->bp.addr == addr){

 			*p = b->hnext;

+			b->bp.addr = -1;

+			b->bp.hash = -1;

 			clrflag(b, Bcached);

 			b->hnext = nil;

 			break;