LU-6051 utils: improve efficiency of lfs_migrate cmp

LU-6051 utils: improve efficiency of lfs_migrate cmp

The file comparison used by lfs_migrate could be more efficient by
using the "cmp -s" option to avoid printing byte-by-byte differences.
This allows faster comparisons via memcmp() and comparing the file
size.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I424538388d7e31609fa1f875128f03fef33ebbe5
Reviewed-on: https://review.whamcloud.com/25878
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>