cifs: allow calling cifs_build_path_to_root on incomplete cifs_sb

It's possible that cifs_mount will call cifs_build_path_to_root on a
newly instantiated cifs_sb. In that case, it's likely that the
master_tlink pointer has not yet been instantiated.

Fix this by having cifs_build_path_to_root take a cifsTconInfo pointer
as well, and have the caller pass that in.
Reported-and-Tested-by: Robbert Kouprie <robbert@exx.nl>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/cifsproto.h

@@ -54,7 +54,8 @@ do {								\
 	     __func__, curr_xid, (int)rc);			\
 } while (0)
 extern char *build_path_from_dentry(struct dentry *);
-extern char *cifs_build_path_to_root(struct cifs_sb_info *cifs_sb);
+extern char *cifs_build_path_to_root(struct cifs_sb_info *cifs_sb,
+					struct cifsTconInfo *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,

fs/cifs/connect.c

@@ -2833,7 +2833,7 @@ cifs_mount(struct super_block *sb, struct cifs_sb_info *cifs_sb,
 	/* check if a whole path (including prepath) is not remote */
 	if (!rc && cifs_sb->prepathlen && tcon) {
 		/* build_path_to_root works only when we have a valid tcon */
-		full_path = cifs_build_path_to_root(cifs_sb);
+		full_path = cifs_build_path_to_root(cifs_sb, tcon);
 		if (full_path == NULL) {
 			rc = -ENOMEM;
 			goto mount_fail_check;

fs/cifs/inode.c

@@ -728,12 +728,12 @@ static const struct inode_operations cifs_ipc_inode_ops = {
 	.lookup = cifs_lookup,
 };
 
-char *cifs_build_path_to_root(struct cifs_sb_info *cifs_sb)
+char *cifs_build_path_to_root(struct cifs_sb_info *cifs_sb,
+				struct cifsTconInfo *tcon)
 {
 	int pplen = cifs_sb->prepathlen;
 	int dfsplen;
 	char *full_path = NULL;
-	struct cifsTconInfo *tcon = cifs_sb_master_tcon(cifs_sb);
 
 	/* if no prefix path, simply set path to the root of share to "" */
 	if (pplen == 0) {
@@ -875,7 +875,7 @@ struct inode *cifs_root_iget(struct super_block *sb, unsigned long ino)
 	char *full_path;
 	struct cifsTconInfo *tcon = cifs_sb_master_tcon(cifs_sb);
 
-	full_path = cifs_build_path_to_root(cifs_sb);
+	full_path = cifs_build_path_to_root(cifs_sb, tcon);
 	if (full_path == NULL)
 		return ERR_PTR(-ENOMEM);