reduce memory allocation in the btrfs direct I/O path v2

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series adds two minor improvements to iomap that allow btrfs
to avoid a memory allocation per read/write system call and another
one per submitted bio.  I also have at last two other pending uses
for the iomap functionality later on, so they are not really btrfs
specific either.

Changes since v1:
 - pass the private data direct to iomap_dio_rw instead of through the
   iocb
 - better document the bio_set in iomap_dio_ops
 - split a patch into three
 - use kcalloc to allocate the checksums

Diffstat:
 fs/btrfs/btrfs_inode.h |   25 --------
 fs/btrfs/ctree.h       |    6 -
 fs/btrfs/file.c        |    6 -
 fs/btrfs/inode.c       |  152 +++++++++++++++++++++++--------------------------
 fs/erofs/data.c        |    2 
 fs/ext4/file.c         |    4 -
 fs/f2fs/file.c         |    4 -
 fs/gfs2/file.c         |    4 -
 fs/iomap/direct-io.c   |   26 ++++++--
 fs/xfs/xfs_file.c      |    6 -
 fs/zonefs/super.c      |    4 -
 include/linux/iomap.h  |   16 ++++-
 12 files changed, 123 insertions(+), 132 deletions(-)