CIFS: Make use of common cifs_build_path_to_root for CIFS and SMB2

because the is no difference here. This also adds support of prefixpath
mount option for SMB2.

Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>
diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
index 07a8ab52..273b349 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -539,8 +539,8 @@
 	char *s, *p;
 	char sep;
 
-	full_path = build_path_to_root(vol, cifs_sb,
-				       cifs_sb_master_tcon(cifs_sb));
+	full_path = cifs_build_path_to_root(vol, cifs_sb,
+					    cifs_sb_master_tcon(cifs_sb));
 	if (full_path == NULL)
 		return ERR_PTR(-ENOMEM);