nfsd: dynamically skip encoded fattr bitmap in _nfsd4_verify

_nfsd4_verify currently skips 3 words from the encoded buffer begining.
With support for 3-word attr bitmaps in nfsd41, nfsd4_encode_fattr
may encode 1, 2, or 3 words, and not always 2 as it used to be, hence
we need to find out where to skip using the encoded bitmap length.

Note: This patch may be applied over pre-nfsd41 nfsd.
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfs4proc.c

@@ -775,7 +775,8 @@ _nfsd4_verify(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	if (status)
 		goto out_kfree;
 
-	p = buf + 3;
+	/* skip bitmap */
+	p = buf + 1 + ntohl(buf[0]);
 	status = nfserr_not_same;
 	if (ntohl(*p++) != verify->ve_attrlen)
 		goto out_kfree;