Clear all of the VFP state in fill_fpregs().

Zero the entire FP register set structure returned for ptrace() if a
thread hasn't used FP registers rather than leaking garbage in the
fp_sr and fp_cr fields.

Reviewed by: emaste, andrew
Approved by: re (rgrimes)
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17140