LU-506 tests: modprobe exportfs for fsfilt_ldiskfs

LU-506 tests: modprobe exportfs for fsfilt_ldiskfs

When trying to load fsfilt_ldiskfs.ko from the build tree, it fails
to load due to the missing exportfs_decode_fh() symbol in RHEL6
kernels.  Force loading of the exportfs.ko module if ldiskfs is used.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iecb13155b69ffc3a227e3df770ca590277cd500c
Reviewed-on: http://review.whamcloud.com/2014
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>