CIFS: add build_path_from_dentry_optional_prefix()

this function does the same thing as add build_path_from_dentry() but
takes a boolean parameter to decide whether or not to prefix the path
with the tree name.

we cannot rely on tcon->Flags & SMB_SHARE_IS_IN_DFS for SMB2 as smb2
code never sets tcon->Flags but it sets tcon->share_flags and it seems
the SMB_SHARE_IS_IN_DFS has different semantics in SMB2: the prefix
shouldn't be added everytime it was in SMB1.

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Acked-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/cifsproto.h

@@ -61,6 +61,8 @@ extern void exit_cifs_idmap(void);
 extern int init_cifs_spnego(void);
 extern void exit_cifs_spnego(void);
 extern char *build_path_from_dentry(struct dentry *);
+extern char *build_path_from_dentry_optional_prefix(struct dentry *direntry,
+						    bool prefix);
 extern char *cifs_build_path_to_root(struct smb_vol *vol,
 				     struct cifs_sb_info *cifs_sb,
 				     struct cifs_tcon *tcon,