The problem with PF_READAHEAD is that if someone does a non-GFP_ATOMIC memory
allocation we can enter page reclaim and then call writepage, while
PF_READAHEAD is set.  The block layer then drops writes or the wrong reads on
the floor.  It can cause data loss.

A fix is complex (well, intrusive).  Given that the readahead code is now
skipping the entire readahead attempt if the queue is congested, the setting
of PF_READAHEAD probably is not doing anything useful anyway, so simply
remove it.