ocfs2: add journal NULL check in ocfs2_checkpoint_inode()

In the Linux kernel, the following vulnerability has been resolved: 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.