ocfs2: don't BUG_ON an invalid journal dinode

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

ocfs2: don't BUG_ON an invalid journal dinode

[BUG]
A fuzzed OCFS2 image can corrupt the current slot journal dinode while
mount is still in progress. The mount path first reports the invalid
journal block and then crashes in shutdown:

kernel BUG at fs/ocfs2/journal.c:1034!
Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
RIP: 0010:ocfs2_journal_toggle_dirty+0x2d6/0x340 fs/ocfs2/journal.c:1034
Call Trace:
ocfs2_journal_shutdown+0x414/0xc30 fs/ocfs2/journal.c:1116
ocfs2_mount_volume fs/ocfs2/super.c:1785 [inline]
ocfs2_fill_super+0x30a9/0x3cd0 fs/ocfs2/super.c:1083
get_tree_bdev_flags+0x38b/0x640 fs/super.c:1698
get_tree_bdev+0x24/0x40 fs/super.c:1721
ocfs2_get_tree+0x21/0x30 fs/ocfs2/super.c:1184
vfs_get_tree+0x9a/0x370 fs/super.c:1758
fc_mount fs/namespace.c:1199 [inline]
do_new_mount_fc fs/namespace.c:3642 [inline]
do_new_mount fs/namespace.c:3718 [inline]
path_mount+0x5b8/0x1ea0 fs/namespace.c:4028
do_mount fs/namespace.c:4041 [inline]
__do_sys_mount fs/namespace.c:4229 [inline]
__se_sys_mount fs/namespace.c:4206 [inline]
__x64_sys_mount+0x282/0x320 fs/namespace.c:4206
...

[CAUSE]
ocfs2_journal_toggle_dirty() used to return -EIO when journal->j_bh no
longer contained a valid dinode, because the startup and shutdown paths
already handled that failure. Commit 10995aa2451a
("ocfs2: Morph the haphazard OCFS2_IS_VALID_DINODE() checks.") changed
the check to a BUG_ON() under the assumption that the journal dinode had
already been validated. That turns an unexpected invalid journal dinode
during mount teardown into a kernel crash instead of a normal mount
failure.

[FIX]
Replace the BUG_ON() with WARN_ON() and return -EIO. This keeps the
invariant warning for debugging, but restores the original behavior of
failing startup or shutdown cleanly instead of panicking the kernel.