os/FileStore: fix to handle readdir error correctly

os/FileStore: fix to handle readdir error correctly

Currently filestore code does not handle readdir error.
As man readdir(3) says, we need to check errno after readdir
returns NULL to determine if error happens or not.

This patch fixes the all readdir() calls to check errono and
handle it appropriately:
 - FileStore.cc ... abort if EIO error happens
 - BtrfsFileStoreBAckend.cc/LFNindex.cc
                ... return error to upper layer

Without this fixes, primary PG could fail to correctly perform
backfill operation and could lead data loss propagation described
in #50558.

Fixes: https://tracker.ceph.com/issues/50558
Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>