an iomap-based direct I/O implementation V3

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series adds a new direct I/O implementation based on the iomap
interface, and switches XFS to use it.

The first two patches are a resend of my earlier series to remove the
XFS iolock.  They are needed for the lockdep assert in the new iomap
code.

The rest implements a new iomap_dio_rw direct I/O implementation and
switches XFS to use it.

Note that this series is on top of a merge of the XFS for-next
tree with the block tree, which has a new helper needed for this
implementation.  This is the block tree it's on top of:

    git://git.kernel.dk/linux-block for-4.10/block

To make everyones life easie I also have a git tree with the merge
plus the patches in this series available here:

    git://git.infradead.org/users/hch/vfs.git iomap-dio.3

Changes since V2:
 - added a few comments to unconfuse reviewers
 - added a iomap_dio_set_error helper
 - dropped support for not filling holes on writes
 - pulled in a more recent block tree (blk_poll -> blk_mq_poll)

Changes since V1:
 - dropped the bio_iov_iter_get_pages patch, now merged in the block tree
 - rebased on top of a merge of the XFS tree with the block tree
 - stopped using WRITE_ODIRECT which has been removed in the block tree