fs: factor out d_mark_tmpfile()

New helper for bcachefs - bcachefs doesn't want the
inode_dec_link_count() call that d_tmpfile does, it handles i_nlink on
its own atomically with other btree updates

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>

fs/dcache.c

@@ -3246,11 +3246,10 @@ void d_genocide(struct dentry *parent)
 	d_walk(parent, parent, d_genocide_kill);
 }
 
-void d_tmpfile(struct file *file, struct inode *inode)
+void d_mark_tmpfile(struct file *file, struct inode *inode)
 {
 	struct dentry *dentry = file->f_path.dentry;
 
-	inode_dec_link_count(inode);
 	BUG_ON(dentry->d_name.name != dentry->d_iname ||
 		!hlist_unhashed(&dentry->d_u.d_alias) ||
 		!d_unlinked(dentry));
@@ -3260,6 +3259,15 @@ void d_tmpfile(struct file *file, struct inode *inode)
 				(unsigned long long)inode->i_ino);
 	spin_unlock(&dentry->d_lock);
 	spin_unlock(&dentry->d_parent->d_lock);
+}
+EXPORT_SYMBOL(d_mark_tmpfile);
+
+void d_tmpfile(struct file *file, struct inode *inode)
+{
+	struct dentry *dentry = file->f_path.dentry;
+
+	inode_dec_link_count(inode);
+	d_mark_tmpfile(file, inode);
 	d_instantiate(dentry, inode);
 }
 EXPORT_SYMBOL(d_tmpfile);

include/linux/dcache.h

@@ -251,6 +251,7 @@ extern struct dentry * d_make_root(struct inode *);
 /* <clickety>-<click> the ramfs-type tree */
 extern void d_genocide(struct dentry *);
 
+extern void d_mark_tmpfile(struct file *, struct inode *);
 extern void d_tmpfile(struct file *, struct inode *);
 
 extern struct dentry *d_find_alias(struct inode *);