LU-9629 utils: fix lfs_migrate for non-root users

LU-9629 utils: fix lfs_migrate for non-root users

Allow lfs_migrate to work with non-root users even when there are
hard-linked files.  The use of "lfs fid2path" is only strictly
needed if "lfs migrate" is not working and the script falls back
to using "rsync" to migrate the hard-linked files.  In the common
case, "lfs migrate" will preserve the links to the file and all
that is needed is "path2fid" to record which FIDs have already
been migrated so that they are not migrated again.

There is no need to track files with only one link, so none of
this FID-handling infrastructure is needed in the common case.

Don't get the mountpoint (via "df") for each hard-linked file within
a single filesystem (which is normally all files).  This is only
needed if files are on different mountpoints, which can be detected
by the device number returned by stat(1) on the file.  Cache the
device number across stat calls, and if it doesn't change then use
the same mountpoint for the fid2path call.

Add named variables to index the fields in the "nlink_type" array to
make it easier to see what is being accessed and avoid bugs.

Fixes: 80a2ff7137d3 ("LU-6051 utils: allow lfs_migrate to handle hard links")
Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: If37d9f73bd1e2ff261fdcfb5248b9e51ae42bd13
Reviewed-on: https://review.whamcloud.com/36383
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>