blk_congestion_wait() will currently not wait if there are no write requests
in flight.  Which is a potential problem if all the dirty data is against NFS
filesystems.

For write(2) traffic against NFS, things work nicely, because writers
throttle in nfs_wait_on_requests().  But for MAP_SHARED dirtyings we need to
avoid spinning in balance_dirty_pages().  So allow callers to fall through to
the explicit sleep in that case.

This will also fix a weird lockup which the reiser4 developers report.  In
that case they have managed to have _all_ inodes against a superblock in
locked state, yet there are no write requests in flight.  Taking a nap in
blk_congestion_wait() in this case will yield the CPU to the threads which
are trying to write out pages.

Also tune up the sleep durations in various callers - 250 milliseconds seems
rather long.