Use public interfaces to manage the nested rate limit send tag.

Each TLS send tag in mlx5 contains a nested rate limit send tag.
Previously, the driver was calling internal functions to manage the
nested tag. Calling free methods directly instead of m_snd_tag_rele()
leaked send tag references and references on the ifp. Changes to use
the ifp methods for the nested tag for other methods are more cosmetic
but do simplify the code.

Reviewed by: gallatin, hselasky
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D26996