ocfs2: update comments in masklog.h

In the mainline ocfs2 code, the interface for masklog is in files under
/sys/fs/o2cb/masklog, but the comments in fs/ocfs2/cluster/masklog.h
reference the old /proc interface.  They are out of date.

This patch modifies the comments in cluster/masklog.h, which also provides
a bash script example on how to change the log mask bits.

Signed-off-by: Coly Li <coly.li@suse.de>
Signed-off-by: Joel Becker <joel.becker@oracle.com>