net: sockets: Simplify common getsockname() implementation

Simplify common `getsockname()` implementation by using VTABLE_CALL()
macro, in the same way as other socket calls do. This additionally
allows to cover the case, when `getsockname()` is not implemnented by
particular socket implementation, preventing the crash.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>