net: pass const to msg_data_left()

The msg_data_left() function doesn't modify the struct msghdr parameter,
so mark it as const. This allows the function to be used with const
references, improving type safety and making the API more flexible.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250408-tcpsendmsg-v3-1-208b87064c28@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>