xfstests: Add ENOSPC Hole Punch Test

xfstests: Add ENOSPC Hole Punch Test

This patch adds a new test 256 that tests that a hole can be punched
even when the disk is full.  Reserved blocks should be used to allow
a punch hole to proceed even when there is not enough blocks to
further fragment the file. To test this, the file system is
fragmented by punching holes in regular intervals and filling the
file system between punches. This will eventually force the file
system to use reserved blocks to proceed with the punch hole
operation.

The work in this patch is a continuation from a previous patch set
that has been partially accepted.

Signed-off-by: Allison Henderson <achender@linux.vnet.ibm.com>
Signed-off-by: Alex Elder <aelder@sgi.com>