Pass the right size to memcpy() when copying the array of FP registers.

The size of the containing structure was passed instead of the size of
the array. This happened to be harmless as the extra word copied is
one we copy in the next line anyway.

Reported by: CHERI (bounds check violation)
Reviewed by: brooks, imp
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D25791