shmem: Fix shmem_rename2()

When renaming onto an existing directory entry, user space expects
the replacement entry to have the same directory offset as the
original one.

Link: https://gitlab.alpinelinux.org/alpine/aports/-/issues/15966
Fixes: a2e45955 ("shmem: stable directory offsets")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/20240415152057.4605-4-cel@kernel.orgSigned-off-by: Christian Brauner <brauner@kernel.org>

fs/libfs.c

@@ -366,6 +366,9 @@ int simple_offset_empty(struct dentry *dentry)
  *
  * Caller provides appropriate serialization.
  *
+ * User space expects the directory offset value of the replaced
+ * (new) directory entry to be unchanged after a rename.
+ *
  * Returns zero on success, a negative errno value on failure.
  */
 int simple_offset_rename(struct inode *old_dir, struct dentry *old_dentry,
@@ -373,8 +376,14 @@ int simple_offset_rename(struct inode *old_dir, struct dentry *old_dentry,
 {
 	struct offset_ctx *old_ctx = old_dir->i_op->get_offset_ctx(old_dir);
 	struct offset_ctx *new_ctx = new_dir->i_op->get_offset_ctx(new_dir);
+	long new_offset = dentry2offset(new_dentry);
 
 	simple_offset_remove(old_ctx, old_dentry);
+
+	if (new_offset) {
+		offset_set(new_dentry, 0);
+		return simple_offset_replace(new_ctx, old_dentry, new_offset);
+	}
 	return simple_offset_add(new_ctx, old_dentry);
 }