bpf, sockmap: Return socket cookie on lookup from syscall

Tooling that populates the SOCK{MAP,HASH} with sockets from user-space
needs a way to inspect its contents. Returning the struct sock * that the
map holds to user-space is neither safe nor useful. An approach established
by REUSEPORT_SOCKARRAY is to return a socket cookie (a unique identifier)
instead.

Since socket cookies are u64 values, SOCK{MAP,HASH} need to support such a
value size for lookup to be possible. This requires special handling on
update, though. Attempts to do a lookup on a map holding u32 values will be
met with ENOSPC error.

Signed-off-by: Jakub Sitnicki <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: John Fastabend <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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