net: mctp: Set SOCK_RCU_FREE

Bind lookup runs under RCU, so ensure that a socket doesn't go away in
the middle of a lookup.

Fixes: 833ef3b9 ("mctp: Populate socket implementation")
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Link: https://patch.msgid.link/20250410-mctp-rcu-sock-v1-1-872de9fdc877@codeconstruct.com.au


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