s3: Fix a memleak in dbwrap_rbt.

s3: Fix a memleak in dbwrap_rbt.

The SMB_MALLOC'ed rbt node data was not free'd on talloc free of
the db context. This is a quick fix using talloc instead of malloc
for allocation of the node data.

Since malloc was originally used for performance reasons, one
might want to reverse to malloc and create a talloc destructor
that walks the tree and frees all the node data if this talloc
approach proves to be too slow..

Michael