minixfs: Use dir_put_page() in minix_unlink() and minix_rename()

... rather than open-coding it there.  Counterpart of the
corresponding fs/sysv commit from Fabio's series...

Reviewed-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>