xfs: wrap xfs_db calls to the test device

Create a _test_xfs_db analogue to _scratch_xfs_db so that we can
encapsulate whatever strange test fs options were fed to us by the test
runner.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/xfs/016

@@ -149,7 +149,7 @@ _log_sunit()
 
 _after_log()
 {
-    xfs_db -r $1 -c "sb" -c "print" | $AWK_PROG '
+    _scratch_xfs_db -r -c "sb" -c "print" | $AWK_PROG '
         /logstart/  { logstart = $3 }
         /logblocks/ { logblocks = $3 }
         END {
@@ -163,7 +163,7 @@ _check_corrupt()
     f="c6c6c6c6"
     echo "*** check for corruption"
     echo "expect $f..." >>$seqres.full
-    xfs_db -r -c "fsblock $2" -c "print" $1 | head | tee -a $seqres.full | \
+    _scratch_xfs_db -r -c "fsblock $2" -c "print" | head | tee -a $seqres.full | \
         grep -q -v "$f $f $f $f $f $f $f $f" && \
             _fail "!!! block $2 corrupted!"
 }