generic/365: Fix false failure when mapping ends with free space

generic/365: Fix false failure when mapping ends with free space

If we have a small FS where the first free space mapping is also the
last mapping of the FS, then the following sub-test fails:

  echo "test whatever came after freesp"
  $XFS_IO_PROG -c "fsmap -d $((freesp_end + 2)) $((freesp_end + 3))" $SCRATCH_MNT

since there is nothing after the freespace. Fix this by punching a 1M
hole in a 3M file to ensure that the first free space is always
surrounded by allocated blocks.

Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Tested-by: Disha Goel <disgoel@linux.ibm.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>