The lwm2m_check_buf_sizes() function was not checking resources of
type LWM2M_RES_TYPE_U16, which could lead to potential overflows
when writing larger data types into U16 resources. Add a case for
LWM2M_RES_TYPE_U16 to properly validate the buffer size.
Fixes#77016
Signed-off-by: Hyunsu Yoon <piedroconti@naver.com>