mptcp: annotate data-races around msk->rmem_fwd_alloc

mptcp: annotate data-races around msk->rmem_fwd_alloc

[ Upstream commit 9531e4a83febc3fb47ac77e24cfb5ea97e50034d ]

msk->rmem_fwd_alloc can be read locklessly.

Add mptcp_rmem_fwd_alloc_add(), similar to sk_forward_alloc_add(),
and appropriate READ_ONCE()/WRITE_ONCE() annotations.

Fixes: 6511882cdd82 ("mptcp: allocate fwd memory separately on the rx and tx path")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>