mprutil: "fix user reply buffer (64)..." warnings

Depending on the card's firmware version, it may return different length
responses for MPI2_FUNCTION_IOC_FACTS. But the first part of the
response contains the length of the rest, so query it first to get the
length and then use that to size the buffer for the full response.

Also, correctly zero-initialize MPI2_IOC_FACTS_REQUEST. It only worked
by luck before.

PR: 264848
Reported by: Julien Cigar <julien@perdition.city>
MFC after: 1 week
Sponsored by: Axcient
Reviewed by: scottl, imp
Differential Revision: https://reviews.freebsd.org/D38739