net: Fix inet_pton when offloading is enabled

When offloading is enabled, a call to inet_pton() results in a call to
zsock_inet_pton() based on its implementation in include/net/socket.h.
This eventually leads to a call to _impl_zsock_inet_pton(), which is
not defined when offloading is enabled.

In this commit, we have chosen to directly call net_addr_pton() in
inet_pton() in the offload case to be efficient, and keep the
implementation as it is when offload is not enabled.

Fixes #12441

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>