tests: fix false dd conv=sparse failure on newer XFS

* tests/dd/sparse.sh: When testing that a hole is created,
use an existing sparse destination file, so that we're
not write extending the file size, and thus avoiding
speculative preallocation which can result in smaller
holes than requested.
Workaround suggested by Brian Foster

THANKS.in

@@ -95,6 +95,7 @@ Bjorn Helgaas                       helgaas@rsn.hp.com
 Bob McCracken                       kerouac@ravenet.com
 Branden Robinson                    branden@necrotic.deadbeast.net
 Brendan O'Dea                       bod@compusol.com.au
+Brian Foster                        bfoster@redhat.com
 Brian Kimball                       bfk@footbag.org
 Brian M. Carlson                    sandals@crustytoothpaste.ath.cx
 Brian Silverman                     bsilverman@conceptxdesign.com

tests/dd/sparse.sh

@@ -61,8 +61,15 @@ if test $(kb_alloc file.in) -gt 3000; then
   dd if=file.in of=file.out bs=2M conv=sparse
   test 2500 -lt $(kb_alloc file.out) || fail=1
 
+  # Note we recreate a sparse file first to avoid
+  # speculative preallocation seen in XFS, where a write() that
+  # extends a file can preallocate some extra space that
+  # a subsequent seek will not convert to a hole.
+  rm -f file.out
+  truncate --size=3M file.out
+
   # Ensure that this 1MiB string of NULs *is* converted to a hole.
-  dd if=file.in of=file.out bs=1M conv=sparse
+  dd if=file.in of=file.out bs=1M conv=sparse,notrunc
   test $(kb_alloc file.out) -lt 2500 || fail=1
 
 fi