d_path: saner calling conventions for __dentry_path()

1) lift NUL-termination into the callers
2) pass pointer to the end of buffer instead of that to beginning.

(1) allows to simplify dentry_path() - we don't need to play silly
games with restoring the leading / of "//deleted" after __dentry_path()
would've overwritten it with NUL.

We also do not need to check if (either) prepend() in there fails -
if the buffer is not large enough, we'll end with negative buflen
after prepend() and __dentry_path() will return the right value
(ERR_PTR(-ENAMETOOLONG)) just fine.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/d_path.c

@@ -326,22 +326,21 @@ char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
 /*
  * Write full pathname from the root of the filesystem into the buffer.
  */
-static char *__dentry_path(const struct dentry *d, char *buf, int buflen)
+static char *__dentry_path(const struct dentry *d, char *p, int buflen)
 {
 	const struct dentry *dentry;
 	char *end, *retval;
 	int len, seq = 0;
 	int error = 0;
 
-	if (buflen < 2)
+	if (buflen < 1)
 		goto Elong;
 
 	rcu_read_lock();
 restart:
 	dentry = d;
-	end = buf + buflen;
+	end = p;
 	len = buflen;
-	prepend(&end, &len, "", 1);
 	/* Get '/' right */
 	retval = end-1;
 	*retval = '/';
@@ -373,27 +372,21 @@ static char *__dentry_path(const struct dentry *d, char *buf, int buflen)
 
 char *dentry_path_raw(const struct dentry *dentry, char *buf, int buflen)
 {
-	return __dentry_path(dentry, buf, buflen);
+	char *p = buf + buflen;
+	prepend(&p, &buflen, "", 1);
+	return __dentry_path(dentry, p, buflen);
 }
 EXPORT_SYMBOL(dentry_path_raw);
 
 char *dentry_path(const struct dentry *dentry, char *buf, int buflen)
 {
-	char *p = NULL;
-	char *retval;
-
-	if (d_unlinked(dentry)) {
-		p = buf + buflen;
-		if (prepend(&p, &buflen, "//deleted", 10) != 0)
-			goto Elong;
-		buflen++;
-	}
-	retval = __dentry_path(dentry, buf, buflen);
-	if (!IS_ERR(retval) && p)
-		*p = '/';	/* restore '/' overriden with '\0' */
-	return retval;
-Elong:
-	return ERR_PTR(-ENAMETOOLONG);
+	char *p = buf + buflen;
+
+	if (unlikely(d_unlinked(dentry)))
+		prepend(&p, &buflen, "//deleted", 10);
+	else
+		prepend(&p, &buflen, "", 1);
+	return __dentry_path(dentry, p, buflen);
 }
 
 static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path *root,