tdb2: fix coalesce race #3

When we're coalescing, we need to drop the lock on the current free list, as
we've enlarged the block and it may now belong in a different list.

Unfortunately (as shown by repeated tdbtorture -n 8) another coalescing run
can do the coalescing while we've dropped the lock.  So for this case, we
use the TDB_COALESCING_MAGIC flag so it doesn't look free.