lindebugfs: Zero the linux_file structure before use.

This avoids clients using garbage values on the stack and makes
debugging easier.

Sponsored by: NVIDIA Networking
Approved by: re (gjb)

(cherry picked from commit 88a29d89ebab1ed60607c3dab73143c3b59a0f83)
(cherry picked from commit b5cc52c21d7d34eae93c996cee09e7d2896afa6e)