remove detritus left by "mm: make read_cache_page synchronous"

gets minix get_dir_page() in sync with its analogs; back in 2007
Nick has switched read_cache_page() and friends to sync behaviour
(i.e.  they wait for the page to get unlocked, check if it's uptodate
and if it isn't return ERR_PTR(-EIO) instead) and removed the
duplicate logics from the callers.  In case of fs/minix/dir.c he'd
removed only half of that...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>