Bluetooth: SDP: Fix issue that RX buf is used for others discovery

In current implementation, the received buffer is incorrectly used for
other SDP discover requests.

Release the RX buffer when the SDP discovery is done. And allocate the
new RX buffer from the pool of the new SDP discovery request.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>