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.

(cherry picked from commit 06afb53bcd7b6a928bc90acb820383302ea8be9e)