Bluetooth: Get rid of __l2cap_get_sock_by_psm()

l2cap_get_sock_by_psm() was the only user of this function, so I merged
both into l2cap_get_sock_by_psm(). The socket lock now should be hold
outside of l2cap_get_sock_by_psm() once we hold and release it inside the
same function now.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>