fix stale delalloc punching for COW I/O v2

[Christoph Hellwig <hch@lst.de>]

Hi all,

this is another fallout from the zoned XFS work, which stresses the XFS
COW I/O path very heavily.  It affects normal I/O to reflinked files as
well, but is very hard to hit there.

The main problem here is that we only punch out delalloc reservations
from the data fork, but COW I/O places delalloc extents into the COW
fork, which means that it won't get punched out forshort writes.

Changes since v1:
 - move the already reviewed iomap prep changes to the beginning in case
   Christian wants to take them ASAP
 - take the invalidate_lock for post-EOF zeroing so that we have a
   consistent locking pattern for zeroing.

Diffstat:
 fs/iomap/buffered-io.c |  139 +++++++++++++++++++++++--------------------------
 fs/xfs/xfs_aops.c      |    4 -
 fs/xfs/xfs_bmap_util.c |   10 ++-
 fs/xfs/xfs_bmap_util.h |    2 
 fs/xfs/xfs_file.c      |  139 ++++++++++++++++++++++++++++---------------------
 fs/xfs/xfs_iomap.c     |   59 +++++++++-----------
 include/linux/iomap.h  |   11 ++-
 7 files changed, 192 insertions(+), 172 deletions(-)