ocfs2: add journal NULL check in ocfs2_checkpoint_inode()

ocfs2: add journal NULL check in ocfs2_checkpoint_inode()

During unmount, ocfs2_journal_shutdown() frees the journal and sets
osb->journal to NULL. Later, when VFS evicts remaining cached inodes,
ocfs2_evict_inode() -> ocfs2_clear_inode() -> ocfs2_checkpoint_inode()
-> ocfs2_ci_fully_checkpointed() dereferences osb->journal, causing a
NULL pointer dereference.

Fix this by adding a NULL check for osb->journal in
ocfs2_checkpoint_inode(). If the journal is NULL, it has already been
fully flushed and destroyed during shutdown, so there is nothing to
checkpoint.

Link: https://lore.kernel.org/20260531131645.3650299-1-joseph.qi@linux.alibaba.com
Reported-by: Farhad Alemi <farhad.alemi@berkeley.edu>
Fixes: da5e7c87827e ("ocfs2: cleanup journal init and shutdown")
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Tested-by: Farhad Alemi <farhad.alemi@berkeley.edu>
Reviewed-by: Heming Zhao <heming.zhao@suse.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Jun Piao <piaojun@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>