nfsd4: memory corruption in numeric_name_to_id()

"id" is type is a uid_t (32 bits) but on 64 bit systems strict_strtoul()
modifies 64 bits of data.  We should use kstrtouint() instead.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>