xfs: allow logical-sector sized O_DIRECT

Some time ago, mkfs.xfs started picking the storage physical
sector size as the default filesystem "sector size" in order
to avoid RMW costs incurred by doing IOs at logical sector
size alignments.

However, this means that for a filesystem made with i.e.
a 4k sector size on an "advanced format" 4k/512 disk,
512-byte direct IOs are no longer allowed.  This means
that XFS has essentially turned this AF drive into a hard
4K device, from the filesystem on up.

XFS's mkfs-specified "sector size" is really just controlling
the minimum size & alignment of filesystem metadata.

There is no real need to tightly couple XFS's minimal
metadata size to the minimum allowed direct IO size;
XFS can continue doing metadata in optimal sizes, but
still allow smaller DIOs for apps which issue them,
for whatever reason.

This patch adds a new field to the xfs_buftarg, so that
we now track 2 sizes:

 1) The metadata sector size, which is the minimum unit and
    alignment of IO which will be performed by metadata operations.
 2) The device logical sector size

The first is used internally by the file system for metadata
alignment and IOs.
The second is used for the minimum allowed direct IO alignment.

This has passed xfstests on filesystems made with 4k sectors,
including when run under the patch I sent to ignore
XFS_IOC_DIOINFO, and issue 512 DIOs anyway.  I also directly
tested end of block behavior on preallocated, sparse, and
existing files when we do a 512 IO into a 4k file on a 
4k-sector filesystem, to be sure there were no unexpected
behaviors.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>