nfsd: Fix sanity check for NFSv4.2 Allocate operations

The NFSv4.2 Allocate operation sanity checks the aa_offset
and aa_length arguments. Since they are assigned to variables
of type off_t (signed) it was possible for them to be negative.
It was also possible for aa_offset+aa_length to exceed OFF_MAX
when stored in lo_end, which is uint64_t.

This patch adds checks for these cases to the sanity check.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D31511