LU-14398 llapi: simplify llapi_fid2path()

LU-14398 llapi: simplify llapi_fid2path()

Simplify llapi_fid2path(). Remove the fid_is_sane() check. Remove the
call to root_ioctl() and use get_root_path() directly.

Lustre-commit: 4cfe77df6f2499effa1644e6ad5a594abb11be23
Lustre-change: https://review.whamcloud.com/41405

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: Ib70b8c9e239c77da8b46408de8341fc8aaf4d1c3
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/41862
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>