net: don't panic on unsupported socket types in newFileFD

[gomaja]

newFileFD accepts any socket whose local address is an IPv4, IPv6 or
Unix address, but (*netFD).addrFunc only maps the socket types this
package supports and returns a nil Addr for any other. newFileFD then
dereferenced that nil Addr, so FileConn, FileListener and
FilePacketConn crashed the program when given, for example, an SCTP
SOCK_SEQPACKET socket.

Return EPROTONOSUPPORT instead, as newFileFD already does for an
address family it does not support.

Fixes #82051

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