fix XFSQA #184 for multiple invocations

Make sure the device node is removed before creating it to allow running the testcase multiple times without recreating the filesystem.

Index: xfstests/184
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RCS file: /cvs/xfs-cmds/xfstests/184,v
retrieving revision 1.1

diff -u -p -r1.1 184
@@ -35,6 +35,7 @@ _supported_os IRIX Linux
 
 _setup_testdir
 
+rm -f $testdir/null
 mknod $testdir/null c 1 3
 chmod 666 $testdir/null
 echo fred > $testdir/null