Remove an extra if_ref().

In r348254, if_snd_tag_alloc() routines were changed to bump the ifp
refcount via m_snd_tag_init(). This function wasn't in the tree at
the time and wasn't updated for the new semantics, so was still doing
a separate bump after if_snd_tag_alloc() returned.

Reviewed by: gallatin
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D26999