Bluetooth: Clean up socket locking in l2cap_sock_recvmsg

This patch cleans up the locking login in l2cap_sock_recvmsg by pairing
up each lock_sock call with a release_sock call. The function already
has a "done" label that handles releasing the socket and returning from
the function so the fix is rather simple.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>