NFS: Fix warning introduced by NFSv4.0 transport blocking patches

When CONFIG_NFS_V4_1 is not enabled, gcc emits this warning:

linux/fs/nfs/nfs4state.c:255:12: warning:
 ‘nfs4_begin_drain_session’ defined but not used [-Wunused-function]
 static int nfs4_begin_drain_session(struct nfs_client *clp)
            ^

Eventually NFSv4.0 migration recovery will invoke this function, but
that has not yet been merged.  Hide nfs4_begin_drain_session()
behind CONFIG_NFS_V4_1 for now.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4state.c

@@ -239,6 +239,8 @@ static void nfs4_end_drain_session(struct nfs_client *clp)
 	}
 }
 
+#if defined(CONFIG_NFS_V4_1)
+
 static int nfs4_drain_slot_tbl(struct nfs4_slot_table *tbl)
 {
 	set_bit(NFS4_SLOT_TBL_DRAINING, &tbl->slot_tbl_state);
@@ -268,8 +270,6 @@ static int nfs4_begin_drain_session(struct nfs_client *clp)
 	return nfs4_drain_slot_tbl(&ses->fc_slot_table);
 }
 
-#if defined(CONFIG_NFS_V4_1)
-
 static int nfs41_setup_state_renewal(struct nfs_client *clp)
 {
 	int status;