Bluetooth: Consolidate code around sk_alloc into a helper function

[ Upstream commit 6bfa273e ]

This consolidates code around sk_alloc into bt_sock_alloc which does
take care of common initialization.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Stable-dep-of: ce60b923 ("Bluetooth: compute LE flow credits based on recvbuf space")
Signed-off-by: Sasha Levin <sashal@kernel.org>