net: avoid an indirect call in ____sys_recvmsg()

CONFIG_RETPOLINE=y made indirect calls expensive.

gcc seems to add an indirect call in ____sys_recvmsg().

Rewriting the code slightly makes sure to avoid this indirection.

Alternative would be to not call sock_recvmsg() and instead
use security_socket_recvmsg() and sock_recvmsg_nosec(),
but this is less readable IMO.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: David Laight <David.Laight@aculab.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>