tdb2: fix traversal bug in free list lock_and_alloc()

tdb2: fix traversal bug in free list lock_and_alloc()

We keep looking even if the current best is exactly right.  This is
really bad, because our smaller free lists hold exactly one size: with
this bug we iterate to the end of the list before hitting the end and
deciding we can use it after all.

Before:
	$ ./speed --transaction --no-sync 2000000
	Adding 2000000 records:  1179 ns (93594224 bytes)
	Finding 2000000 records:  694 ns (93594224 bytes)
	Missing 2000000 records:  429 ns (93594224 bytes)
	Traversing 2000000 records:  519 ns (93594224 bytes)
	Deleting 2000000 records:  896 ns (93594224 bytes)
	Re-adding 2000000 records:  1129 ns (93594224 bytes)
	Appending 2000000 records:  1713 ns (182801232 bytes)
	Churning 2000000 records:  32612 ns (182801232 bytes)

After:
	$ ./speed --transaction --no-sync 2000000
	Adding 2000000 records:  1195 ns (93594224 bytes)
	Finding 2000000 records:  719 ns (93594224 bytes)
	Missing 2000000 records:  429 ns (93594224 bytes)
	Traversing 2000000 records:  523 ns (93594224 bytes)
	Deleting 2000000 records:  901 ns (93594224 bytes)
	Re-adding 2000000 records:  1032 ns (93594224 bytes)
	Appending 2000000 records:  1711 ns (182801232 bytes)
	Churning 2000000 records:  3233 ns (182801232 bytes)