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blk: don't reinitialize log block with new address

when the log block was initialized, we cached it with
an address; reinitializing it means that we have a
pointer to it in the cache, but because the address
has changed, we can never look it up.

this means that if we try to pluck it, we will leave
it in place, and if we insert it again, we can end up
with a cycle in the hash table, which will cause bad
things and infinite loops to happen.

as a result, we need to allocate a new block.

--- a/blk.c

+++ b/blk.c

@@ -464,6 +464,10 @@

 	 * so we don't record this block as

 	 * available when we compress the log.

 	 */

+	if((b = cachepluck()) == nil){

+		free(log);

+		return -1;

+	}

 	if((ba = blkalloc_lk(a, 1)) == -1){

 		free(log);

 		return -1;

--- a/cache.c

+++ b/cache.c

@@ -90,6 +90,7 @@

 		return;

 	}

 	setflag(b, Bcached);

+	b->cached = getcallerpc(&b);

 	b->hnext = bkt->b;

 	bkt->b = b;

 	unlock(bkt);

@@ -115,6 +116,7 @@

 			b->bp.addr = -1;

 			b->bp.hash = -1;

 			clrflag(b, Bcached);

+			b->uncached = getcallerpc(&addr);

 			b->hnext = nil;

 			break;

 		}

@@ -166,10 +168,9 @@

 	assert(b->magic == Magic);

 	assert(b->ref == 0);

 	cachedel(b->bp.addr);

+	assert(!checkflag(b, Bcached));

 	lrudel(b);

 	b->flag = 0;

-	b->bp.addr = -1;

-	b->bp.hash = -1;

 	b->lasthold = 0;

 	b->lastdrop = 0;

 	b->freed = 0;