tc: m_action: Improve conversion to C99 style initializers

tc: m_action: Improve conversion to C99 style initializers

This improves my initial change in the following points:

- Flatten embedded struct's initializers.
- No need to initialize variables to zero as the key feature of C99
  initializers is to do this implicitly.
- By relocating the declaration of struct rtattr *tail, it can be
  initialized at the same time.

Fixes: a0a73b298a579 ("tc: m_action: Use C99 style initializers for struct req")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: David Ahern <dsa@cumulusnetworks.com>