net: mctp: Prevent duplicate binds

Disallow bind() calls that have the same arguments as existing bound
sockets.  Previously multiple sockets could bind() to the same
type/local address, with an arbitrary socket receiving matched messages.

This is only a partial fix, a future commit will define precedence order
for MCTP_ADDR_ANY versus specific EID bind(), which are allowed to exist
together.

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Link: https://patch.msgid.link/20250710-mctp-bind-v4-2-8ec2f6460c56@codeconstruct.com.au


Signed-off-by: Paolo Abeni <pabeni@redhat.com>