vsock: Refactor vsock_*_getsockopt to resemble sock_getsockopt

In preparation for sharing the implementation of sock_get_timeout.

Signed-off-by: Richard Palethorpe <rpalethorpe@suse.com>
Cc: Richard Palethorpe <rpalethorpe@richiejp.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>