Test 091 fails on sector size != 512 bytes

Test 091 fails on sector size != 512 bytes

Test 091 assumes a direct I/O alignment of 512 bytes,
a hold over from 2.4 kernels. On 2.6. kernels, direct
I/O needs to be aligned to the sector size the filesystem
was mkfs'd with.
Merge of master-melb:xfs-cmds:30760a by kenmcd.

  Teach 091 about 2.6 kernels and grab the sector size from the
  xfs_info output.