LU-8837 utils: move lustre_disk_data back to lustre_disk.h

LU-8837 utils: move lustre_disk_data back to lustre_disk.h

This patch moves struct lustre_disk_data from mount_utils.h
back to lustre_disk.h so that it can be used in other codes
without including mount_utils.h.

Fixes: d62efba975d2 ("LU-8837 utils: make tools lightweight for lustre clients")
Change-Id: I589da2710e3cbe7d93a59928143f2b5cac955e6e
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44829
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>