enable iomap dio write completions from interrupt context

[Christoph Hellwig <hch@lst.de>]

Hi all,

Currently iomap defers all write completions to interrupt context.  This
was based on my assumption that no one cares about the latency of those
to simplify the code vs the old direct-io.c.  It turns out someone cared,
as Avi reported a lot of context switches with ScyllaDB, which at least
in older kernels with workqueue scheduling issues caused really high
tail latencies.

Fortunately allowing the direct completions is pretty easy with all the
other iomap changes we had since.

While doing this I've also found dead code which gets removed (patch 1)
and an incorrect assumption in zonefs that read completions are called
in user context, which it assumes for it's error handling.  Fix this by
always calling error completions from user context (patch 2).

Against the vfs/vfs-6.19.iomap branch.

Diffstat:
 Documentation/filesystems/iomap/operations.rst |    4 
 fs/backing-file.c                              |    6 -
 fs/iomap/direct-io.c                           |  149 +++++++++++--------------
 include/linux/fs.h                             |   43 +------
 io_uring/rw.c                                  |   16 --
 5 files changed, 81 insertions(+), 137 deletions(-)