mkdir: allow to create directory whose path contains symlink

Fixes a regression caused by 3008a6f21c.

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.

src/basic/mkdir.c

@@ -162,7 +162,7 @@ int mkdir_p_internal(const char *prefix, const char *path, mode_t mode, uid_t ui
 
         assert(_mkdirat != mkdirat);
 
-        r = mkdir_parents_internal(prefix, path, mode, uid, gid, flags, _mkdirat);
+        r = mkdir_parents_internal(prefix, path, mode, uid, gid, flags | MKDIR_FOLLOW_SYMLINK, _mkdirat);
         if (r < 0)
                 return r;