net: sockets: Cleanup socket properly if POSIX API is enabled

The sock_obj_core_dealloc() was not called if close() is called
instead of zsock_close(). This happens if POSIX API is enabled.

Fix this by calling zvfs_close() from zsock_close() and then
pass the socket number to zsock_close_ctx() so that the cleanup
can be done properly.

Reported-by: Andreas Ålgård <aal@ixys.no>
Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>