tcp: remove invalid __rcu annotation

struct tcp_fastopen_context has a field named tfm, which is a pointer
to a crypto_cipher structure.

It currently has a __rcu annotation, which is not needed at all.

tcp_fastopen_ctx is the pointer fetched by rcu_dereference(), but once
we have a pointer to current tcp_fastopen_context, we do not use/need
rcu_dereference() to access tfm.

This fixes a lot of sparse errors like the following :

net/ipv4/tcp_fastopen.c:21:31: warning: incorrect type in argument 1 (different address spaces)
net/ipv4/tcp_fastopen.c:21:31:    expected struct crypto_cipher *tfm
net/ipv4/tcp_fastopen.c:21:31:    got struct crypto_cipher [noderef] <asn:4>*tfm

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jerry Chu <hkchu@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>