nfs_clrpcops.c: Add sanity checks for the slot cnts

The reply to CreateSession includes the slot cnt for
both fore and back slots. It should never be larger
than the argument specified and the fore slot cnt
should always be at least 1.

Without this patch, the replied slot cnts were not
being sanity checked.

While here, replace 64 with NFSV4_SLOTS (which is 64).

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: emaste, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53363