nfsd: wrong index used in inner loop

We must not use dummy for index.
After the first index, READ32(dummy) will change dummy!!!!

Signed-off-by: Mi Jinlong <mijinlong@cn.fujitsu.com>
[bfields@redhat.com: Trond points out READ_BUF alone is sufficient.]
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>