ovl: fix UAF in ovl_do_mkdir() debug print

ovl_do_mkdir() prints the input dentry with %pd after vfs_mkdir().
Since commit fe497f0759e0 ("VFS: change vfs_mkdir() to unlock on
failure."), vfs_mkdir() calls end_creating() on the input dentry on
failure and may replace it on success, so the post-call %pd can
use-after-free the dentry when CONFIG_OVERLAY_FS_DEBUG is enabled.

Print the dentry before the call and only the result afterward.

Reported-by: syzbot+ced26b784bf977d223dd@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ced26b784bf977d223dd
Fixes: fe497f0759e0 ("VFS: change vfs_mkdir() to unlock on failure.")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://patch.msgid.link/20260921104013.40475-1-amir73il@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>