Fix: Check return value of fpathconf

Current glibc has a bug in fpathconf(fd, _PC_NAME_MAX) where it will
fail with a 32bit userland on a 64bit kernel and where the filesystem
has a large block count, see glibc bug #18675.

In any case, we should check this return value because on a failure we
we don't allocate enough memory for dirent and then overflow on the
readdir_r call.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>