net: fix documentation for kernel_getsockname

Fixes return value documentation of kernel_getsockname()
and kernel_getpeername() functions.

The previous documentation wrongly specified that the return
value is 0 in case of success, however sock->ops->getname returns
the length of the address in bytes in case of success.

Signed-off-by: Alex Maydanik <alexander.maydanik@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>