arm: Zero padding in mcontext_vfp_t in get_vfpcontext().

This can leak kernel stack data otherwise.

Reviewed by: imp, markj
Sponsored by: The University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D34594

(cherry picked from commit b234b8d911cdb4fe53032fde0cb8611ae9d0b81d)