Fix TSI datagram socket handling

[mtjhrc]

The TSI datagram implementation had a bug where err == 0 was used to check sendmsg success, but sendmsg returns the number of bytes sent (positive value) on success. This caused the socket to never properly transition to S_VSOCK state, though datagrams still worked because S_HYBRID always fell through to the vsock path.

Changes

setsockopt fixes:

• Add a defensive null pointer check before calling isocket->ops->setsockopt to prevent kernel panic when the underlying socket doesn't implement setsockopt
• Add SOL_SOCKET level handling in tsi_dgram_setsockopt (was missing, unlike the stream version)

sendmsg fixes:

• Fix state transition condition from err == 0 to err > 0 - sendmsg returns bytes sent (positive) on success, never zero
• Add local UDP listener detection via tsi_has_udp_listener() before proxying to host, enabling proper guest-to-guest localhost UDP communication

recvmsg fixes:

• Make S_HYBRID state actually try receiving from inet socket instead of returning -ENOTCONN
• Restore msg_namelen after receiving proxied datagrams via vsock (fixes IPv6 UDP)

This fixes the newly added tests in containers/libkrun#472

[slp]

This is looking really good, thanks @mtjhrc. Could you please rebase on top of current main?

[slp]

SEV and TDX builds fail with:

ld: net/tsi/af_tsi.o: in function `tsi_dgram_sendmsg':
af_tsi.c:(.text+0x2ff2): undefined reference to `__udp6_lib_lookup'

That's because TSI now depends in INET6. Please update net/tsi/Kconfig to reflect this, and update both config-libkrunfw-sev_x86_64 and config-libkrunfw-tdx_x86_64 accordingly.

[slp]

LGTM, thanks!