LU-18082 utils: fid2path support for path exceeding PATH_MAX

LU-18082 utils: fid2path support for path exceeding PATH_MAX

PATH_MAX affects the path length of a single pathname, but it is
possible to exceed this by eg. creating a subdirectory tree, then cd
into it, then create a deep tree below this using a relative pathname.

So fix ll_fid2path and lfs_fid2path to not limit the output buffer to
PATH_MAX.

Add sanity test_154i to exercise this.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ic3a0e3ac1e806e45eb4c72c571f33404c60d0df5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55897
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Stephane Thiell <sthiell@stanford.edu>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>