RFC: add STATX_DIO_READ_ALIGN

[Christoph Hellwig <hch@lst.de>]

Hi all,

file systems that write out of place usually require different alignment
for direct I/O writes than what they can do for reads.  This series tries
to address this by yet another statx field.

Note that the code is completely untested - I wrote it and got preempted
and only sent this out because Hongbo Li brought the issue up in the
nilfs2 context.  I've just started a vacation so I'm unlikely to get
back to it any time soon, but if someone wants to take the work over
go for it.  I'll probably answer to email at least every other day or
so.

Diffstat:
 fs/stat.c                 |    1 
 fs/xfs/xfs_iops.c         |   37 +++++++++++++----
 include/linux/stat.h      |    1 
 include/uapi/linux/stat.h |   99 ++++++++++++++++++++++++++++++++++------------
 4 files changed, 106 insertions(+), 32 deletions(-)