ocfs2: add feature document for online file check

This document will describe OCFS2 online file check feature.  OCFS2 is
often used in high-availaibility systems.  However, OCFS2 usually
converts the filesystem to read-only when encounters an error.  This may
not be necessary, since turning the filesystem read-only would affect
other running processes as well, decreasing availability.

Then, a mount option (errors=continue) is introduced, which would return
the -EIO errno to the calling process and terminate furhter processing
so that the filesystem is not corrupted further.  The filesystem is not
converted to read-only, and the problematic file's inode number is
reported in the kernel log.  The user can try to check/fix this file via
online filecheck feature.

Signed-off-by: Gang He <ghe@suse.com>
Cc: Mark Fasheh <mfasheh@suse.de>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Joseph Qi <joseph.qi@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>