NFS: silence a harmless uninitialized variable warning

kstrtoul() can return -ERANGE so Smatch complains that "num" can be
uninitialized.  We check that it's within bounds so it's not a huge
deal.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>