tcp: Remove CMSG magic numbers for tcp_recvmsg().

At present, tcp_recvmsg() uses flags to track if any CMSGs are pending
and what those CMSGs are. These flags are currently magic numbers,
used only within tcp_recvmsg().

To prepare for receive timestamp support in tcp receive zerocopy,
gently refactor these magic numbers into enums.

Signed-off-by: Arjun Roy <arjunroy@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>