LU-976 tests: skip MMP test for non-ldiskfs backfs

Skip the Multi-Mount Protection tests for non-ldiskfs backing
filesystems.  MMP is currently only implemented for ldiskfs,
so until that changes this test cannot pass for any other FSTYPE.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I4dc9936fa6f34b4dc4cc465a6352daae2198788f
Reviewed-on: http://review.whamcloud.com/1934
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>