LU-6051 utils: allow lfs_migrate to migrate links

LU-6051 utils: allow lfs_migrate to migrate links

When running on a server that supports "lfs migrate", it is possible
to migrate hard links transparently.  This is still not possible for
servers that do not support "lfs migrate" (layout swap) so we still
need to keep the hard-link check if "lfs migrate" fails.

Add test for migrating many hardlinks on supported kernels.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I7fa1c70611aa7dd61f822844d5fe339f730640e5
Reviewed-on: https://review.whamcloud.com/27553
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Steve Guminski <stephenx.guminski@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>