nfsd: don't ignore high bits of copy count

Note size_t is 32-bit on a 32-bit architecture, but cp_count is defined
by the protocol to be 64 bit, so we could be turning a large copy into a
0-length copy here.

Reported-by:  <radchenkoy@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>