net: constify some inline functions in sock.h

Despite these inline functions having full visibility to the compiler
at compile time, they still strip const from passed pointers.
This change allows for functions in various network drivers to be marked as
const that could not be marked const before.

Signed-off-by: Peter Lafreniere <pjlafren@mtu.edu>
Link: https://lore.kernel.org/r/20220606113458.35953-1-pjlafren@mtu.edu


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