setmntent() doesn't support root_fd, but manual parsing of
/proc/mounts is fragile, and actually buggy for very long mount lines
(see open-mpi/hwloc#142 (comment)).
Since we only openat("/proc/mounts") there, just manually concatenate
the fsroot_path and use setmntent().
Thanks to Nathan Hjelm for the report.
(Cherry-picked from open-mpi/hwloc@d2d07b9a22)
Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>
15007b4e2b