nfsd4: add a helper function to decide if stateid is delegation

Make this check self-documenting.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 909a7a4..d555585 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -2048,6 +2048,11 @@
 	return nfs_ok;
 }
 
+static int is_delegation_stateid(stateid_t *stateid)
+{
+	return stateid->si_fileid == 0;
+}
+
 /*
 * Checks for stateid operations
 */
@@ -2073,7 +2078,7 @@
 		goto out;
 
 	status = nfserr_bad_stateid;
-	if (!stateid->si_fileid) { /* delegation stateid */
+	if (is_delegation_stateid(stateid)) {
 		dp = find_delegation_stateid(ino, stateid);
 		if (!dp)
 			goto out;