current->flags:PF_SYNC was a hack I added because I didn't want to
change all ->writepage implementations.

It's foul.  And it means that if someone happens to run direct page
reclaim within the context of (say) sys_sync, the writepage invokations
from the VM will be treated as "data integrity" operations, not "memory
cleansing" operations, which would cause latency.

So the patch removes PF_SYNC and adds an extra arg to a_ops->writepage.
 It is the `writeback_control' structure which contains the full context
information about why writepage was called.

The initial version of this patch just passed in a bare `int sync', but
the XFS team need more info so they can perform writearound from within
page reclaim.

The patch also adds writeback_control.for_reclaim, so writepage
implementations can inspect that to work out the call context rather
than peeking at current->flags:PF_MEMALLOC.