LU-14398 llapi: add llapi_fid2path_at()

Add llapi_fid2path_at() which works like llapi_fid2path() takes an
open FD on the moint point instead of a 'fsname or dirirectory path'
and a const struct lu_fid * instead of a const char *.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I76234bc28de231587b65c5d866954441e0893aac
Reviewed-on: https://review.whamcloud.com/41406
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>