nfsd: avoid undefined signed overflow

In C, signed integer overflow results in undefined behavior, but unsigned
overflow wraps around. So do the subtraction first, then cast to signed.

Reported-by: Joakim Tjernlund <joakim.tjernlund@transmode.se>
Signed-off-by: Jim Rees <rees@umich.edu>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>