xfstests: xfs_tosspages() test addition

xfstests: xfs_tosspages() test addition

Tests the XFS_IOC_ZERO_RANGE ioctl by way of the "xfs_io zero" utility
to ensure it is tossing the expected ranges.

The ranges tested are [0,1] [0,4095] [0,4096] [0,4097] [4095,8191]
  [4095,8192] [4095,8193] [4096,8192] [1024,4096]

Signed-off-by: Andrew Dahl <adahl@sgi.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Mark Tinguely <tinguely@sgi.com>