exfat: remove argument 'p_dir' from exfat_add_entry()

The output of argument 'p_dir' of exfat_add_entry() is not used
in either exfat_mkdir() or exfat_create(), remove the argument.

Code refinement, no functional changes.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
Reviewed-by: Daniel Palmer <daniel.palmer@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>