jfs: fix invalid free of JFS_IP(ipimap)->i_imap in diUnmount

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

jfs: fix invalid free of JFS_IP(ipimap)->i_imap in diUnmount

syzbot found an invalid-free in diUnmount:

BUG: KASAN: double-free in slab_free mm/slub.c:3661 [inline]
BUG: KASAN: double-free in __kmem_cache_free+0x71/0x110 mm/slub.c:3674
Free of addr ffff88806f410000 by task syz-executor131/3632

CPU: 0 PID: 3632 Comm: syz-executor131 Not tainted 6.1.0-rc7-syzkaller-00012-gca57f02295f1 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x1b1/0x28e lib/dump_stack.c:106
print_address_description+0x74/0x340 mm/kasan/report.c:284
print_report+0x107/0x1f0 mm/kasan/report.c:395
kasan_report_invalid_free+0xac/0xd0 mm/kasan/report.c:460
____kasan_slab_free+0xfb/0x120
kasan_slab_free include/linux/kasan.h:177 [inline]
slab_free_hook mm/slub.c:1724 [inline]
slab_free_freelist_hook+0x12e/0x1a0 mm/slub.c:1750
slab_free mm/slub.c:3661 [inline]
__kmem_cache_free+0x71/0x110 mm/slub.c:3674
diUnmount+0xef/0x100 fs/jfs/jfs_imap.c:195
jfs_umount+0x108/0x370 fs/jfs/jfs_umount.c:63
jfs_put_super+0x86/0x190 fs/jfs/super.c:194
generic_shutdown_super+0x130/0x310 fs/super.c:492
kill_block_super+0x79/0xd0 fs/super.c:1428
deactivate_locked_super+0xa7/0xf0 fs/super.c:332
cleanup_mnt+0x494/0x520 fs/namespace.c:1186
task_work_run+0x243/0x300 kernel/task_work.c:179
exit_task_work include/linux/task_work.h:38 [inline]
do_exit+0x664/0x2070 kernel/exit.c:820
do_group_exit+0x1fd/0x2b0 kernel/exit.c:950
__do_sys_exit_group kernel/exit.c:961 [inline]
__se_sys_exit_group kernel/exit.c:959 [inline]
__x64_sys_exit_group+0x3b/0x40 kernel/exit.c:959
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
[...]

JFS_IP(ipimap)->i_imap is not setting to NULL after free in diUnmount.
If jfs_remount() free JFS_IP(ipimap)->i_imap but then failed at diMount().
JFS_IP(ipimap)->i_imap will be freed once again.
Fix this problem by setting JFS_IP(ipimap)->i_imap to NULL after free.