LU-1895 tests: don't fail mmp test_5 due to race

LU-1895 tests: don't fail mmp test_5 due to race

In the mmp.sh test_5() mount_after_unmount() testing, it is possible
that the first filesystem unmounts successfully before the second
one starts, and there is no contention for the MMP block.

This caused the test to fail on a regular basis.  However, there is
still value in running this test, since non-MMP race conditions have
previously been seen in this area (OBD device refcount, etc).

Make mount_after_unmount() more robust, only failing if the first
filesystem is still mounted at the same time as the second one.

Author: Andreas Dilger <adilger@whamcloud.com>

Test-Parameters: trivial mdtfilesystemtype=ldiskfs failover=true ostfilesystemtype=ldiskfs osscount=2 mdscount=2 mdtcount=1 austeroptions=-R iscsi=1 testlist=mmp
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: James Nunez <jnunez@whamcloud.com>
Change-Id: I186b9ce0a5a0e1ed6f2b46895fec4a32e73ebbe5