minixfs: change the signature of dir_get_page()

Change the signature of dir_get_page() in order to prepare this function
to the conversion to the use of kmap_local_page(). Change also those call
sites which are required to adjust to the new signature.

Essentially a copy of the corresponding fs/sysv commit by
Fabio M. De Francesco <fmdefrancesco@gmail.com>

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