fat_make_attrs() - makes FAT attributes from inode.
fat_make_mode() - makes mode_t from FAT attributes.
fat_save_attrs() - saves FAT attributes to inode.
Then this replaces: MSDOS_MKMODE() by fat_make_mode(), fat_attr() by
fat_make_attrs(), ->i_attrs = attr & ATTR_UNUSED by fat_save_attrs().
And for root inode, those is used with ATTR_DIR instead of bogus
ATTR_NONE.