As Jens reported, we'd better assign REQ_RAHEAD to bio by the fact that
->readpages is called only from read-ahead.
In Documentation/filesystems/vfs.txt,
readpages: called by the VM to read pages associated with the address_space
object. This is essentially just a vector version of
readpage. Instead of just one page, several pages are
requested.
readpages is only used for read-ahead, so read errors are
ignored. If anything goes wrong, feel free to give up.