NFSD: pass null terminated buf to kstrtouint()

The 'buf' is prepared with null termination with intention of using it for
this purpose, but 'name' is passed instead!

Signed-off-by: Malahal Naineni <malahal@us.ibm.com>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>