mctp: return -ENOPROTOOPT for unknown getsockopt options

In mctp_getsockopt(), unrecognized options currently return -EINVAL.
In contrast, mctp_setsockopt() returns -ENOPROTOOPT for unknown
options.

Update mctp_getsockopt() to also return -ENOPROTOOPT for unknown
options. This aligns the behavior of getsockopt() and setsockopt(),
and matches the standard kernel socket API convention for handling
unsupported options.

Fixes: 99ce45d5 ("mctp: Implement extended addressing")
Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Link: https://patch.msgid.link/20250902102059.1370008-1-alok.a.tiwari@oracle.com


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