ocfs2: load global_inode_alloc

Set global_inode_alloc as OCFS2_FIRST_ONLINE_SYSTEM_INODE, that will
make it load during mount.  It can be used to test whether some
global/system inodes are valid.  One use case is that nfsd will test
whether root inode is valid.

Link: http://lkml.kernel.org/r/20200616183829.87211-3-junxiao.bi@oracle.com
Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Gang He <ghe@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Jun Piao <piaojun@huawei.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>