LU-930 utils: update lfs_migrate example usage

Update the lfs_migrate example usage to exclude files created within
the past two days.  This makes it more clear that the current script
needs to take care to avoid migrating files that may still be modified
by applications.  While two day old files are not guaranteed to be
unused, it definitely would avoid the vast majority of cases of files
that were just created and are being written to by running apps.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: If7c06ef3b0688db492930a56d7ba5ad4599b04e2
Reviewed-on: http://review.whamcloud.com/4481
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Richard Henwood <richard.henwood@intel.com>
Reviewed-by: Li Wei <wei.g.li@intel.com>