btrfs: dio iomap DSYNC workaround

iomap dio will run generic_write_sync() for us if the iocb is DSYNC.
This is problematic for us because of 2 reasons:

1. we hold the inode_lock() during this operation, and we take it in
   generic_write_sync()
2. we hold a read lock on the dio_sem but take the write lock in fsync

Since we don't want to rip out this code right now, but reworking the
locking is a bit much to do at this point, work around this problem with
this masterpiece of a patch.

First, we clear DSYNC on the iocb so that the iomap stuff doesn't know
that it needs to handle the sync.  We save this fact in
current->journal_info, because we need to see do special things once
we're in iomap_begin, and we have no way to pass private information
into iomap_dio_rw().

Next we specify a separate iomap_dio_ops for sync, which implements an
->end_io() callback that gets called when the dio completes.  This is
important for AIO, because we really do need to run generic_write_sync()
if we complete asynchronously.  However if we're still in the submitting
context when we enter ->end_io() we clear the flag so that the submitter
knows they're the ones that needs to run generic_write_sync().

This is meant to be temporary.  We need to work out how to eliminate the
inode_lock() and the dio_sem in our fsync and use another mechanism to
protect these operations.

Tested-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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