xfstests: Change mount method for generic/258

xfstests: Change mount method for generic/258

Use the built-in _test_mount function from xfstests so it will use
the correct mount options for xfstests.  The script used a simple
umount-and-mount sequence, which caused a test failure on an XFS
filesystem that used both realtime and external log devices.

Signed-off-by: Michael L. Semon <mlsemon35@gmail.com>
Reviewed-by: Eric Sandeen <sandeen@rehat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>