When a throttled writer is performing writeback, and it encounters an inode
which is already under writeback it is forced to wait on the inode.  So that
process sleeps until whoever is writing it out finishes the writeout.

Which is OK - we want to throttle that process, and another process is
currently pumping data at the disk anyway.

But in one situations the delays are excessive.  If one process is performing
a huge linear write, other processes end up waiting for a very long time
indeed.  It appears that this is because the writing process just keeps on
hogging the CPU, returning to userspace, generating more dirty data, writing
it out, sleeping in get_request_wait, etc.  All other throttled dirtiers get
starved.

So just remove the wait altogether if it is just a memory-cleansing writeout.
 The calling process will then throttle in balance_dirty_pages()'s call to
blk_congestion_wait().