mkdir: allow to create directory whose path contains symlink

Fixes a regression caused by 3008a6f2.

Before the commit, when `mkdir_parents_internal()` is called from `mkdir_p()`,
it uses `_mkdir()` as `flag` is zero. But after the commit, `mkdir_safe_internal()`
is always used. Hence, if the path contains a symlink, it fails with -ENOTDIR.

To fix the issue, this makes `mkdir_p()` calls `mkdir_parents_internal()` with
MKDIR_FOLLOW_SYMLINK flag.

Fixes #22334.

(cherry picked from commit 5117059e)