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>

fs/cifs/cifsproto.h

@@ -60,6 +60,9 @@ extern int init_cifs_idmap(void);
 extern void exit_cifs_idmap(void);
 extern void cifs_destroy_idmaptrees(void);
 extern char *build_path_from_dentry(struct dentry *);
+extern char *cifs_build_path_to_root(struct smb_vol *vol,
+				     struct cifs_sb_info *cifs_sb,
+				     struct cifs_tcon *tcon);
 extern char *build_wildcard_path_from_dentry(struct dentry *direntry);
 extern char *cifs_compose_mount_options(const char *sb_mountdata,
 		const char *fullpath, const struct dfs_info3_param *ref,