ovl: stop using d_alloc_anon()/d_instantiate_anon()

Commit f9c34674 ("vfs: factor out helpers d_instantiate_anon() and
d_alloc_anon()") was introduced so overlayfs could initialize a non-dir
disconnected overlay dentry before overlay inode is attached to it.

Since commit ("0af950f5 ovl: move ovl_entry into ovl_inode"), all
ovl_obtain_alias() can do is set DCACHE_OP_*REVALIDATE flags in ->d_flags
and OVL_E_UPPER_ALIAS flag in ->d_fsdata.

The DCACHE_OP_*REVALIDATE flags and OVL_E_UPPER_ALIAS flag are irrelevant
for a disconnected non-dir dentry, so it is better to use d_obtain_alias()
instead of open coding it.
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/overlayfs/export.c

@@ -289,7 +289,6 @@ static struct dentry *ovl_obtain_alias(struct super_block *sb,
 {
 	struct dentry *lower = lowerpath ? lowerpath->dentry : NULL;
 	struct dentry *upper = upper_alias ?: index;
-	struct dentry *dentry;
 	struct inode *inode = NULL;
 	struct ovl_entry *oe;
 	struct ovl_inode_params oip = {
@@ -320,27 +319,7 @@ static struct dentry *ovl_obtain_alias(struct super_block *sb,
 	if (upper)
 		ovl_set_flag(OVL_UPPERDATA, inode);
 
-	dentry = d_find_any_alias(inode);
-	if (dentry)
-		goto out_iput;
-
-	dentry = d_alloc_anon(inode->i_sb);
-	if (unlikely(!dentry))
-		goto nomem;
-
-	if (upper_alias)
-		ovl_dentry_set_upper_alias(dentry);
-
-	ovl_dentry_init_reval(dentry, upper, OVL_I_E(inode));
-
-	return d_instantiate_anon(dentry, inode);
-
-nomem:
-	dput(dentry);
-	dentry = ERR_PTR(-ENOMEM);
-out_iput:
-	iput(inode);
-	return dentry;
+	return d_obtain_alias(inode);
 }
 
 /* Get the upper or lower dentry in stack whose on layer @idx */