mkfs.xfs: properly handle physical sector size

mkfs.xfs: properly handle physical sector size

This splits the fs_topology structure "sectorsize" into
logical & physical, and gets both via blkid_get_topology().

This primarily allows us to default to using the physical
sectorsize for mkfs's "sector size" value, the fundamental
size of any IOs the filesystem will perform.

We reduce mkfs.xfs's "sector size" to logical if
a block size < physical sector size is specified.
This is suboptimal, but permissable.

For block size < sector size, differentiate the error
message based on whether the sector size was manually
specified, or deduced.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>