md/bitmap: don't set sb values if can't pass sanity check

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

md/bitmap: don't set sb values if can't pass sanity check

If bitmap area contains invalid data, kernel will crash then mdadm
triggers "Segmentation fault".
This is cluster-md speical bug. In non-clustered env, mdadm will
handle broken metadata case. In clustered array, only kernel space
handles bitmap slot info. But even this bug only happened in clustered
env, current sanity check is wrong, the code should be changed.

How to trigger: (faulty injection)

dd if=/dev/zero bs=1M count=1 oflag=direct of=/dev/sda
dd if=/dev/zero bs=1M count=1 oflag=direct of=/dev/sdb
mdadm -C /dev/md0 -b clustered -e 1.2 -n 2 -l mirror /dev/sda /dev/sdb
mdadm -Ss
echo aaa > magic.txt
== below modifying slot 2 bitmap data ==
dd if=magic.txt of=/dev/sda seek=16384 bs=1 count=3 <== destroy magic
dd if=/dev/zero of=/dev/sda seek=16436 bs=1 count=4 <== ZERO chunksize
mdadm -A /dev/md0 /dev/sda /dev/sdb
== kernel crashes. mdadm outputs "Segmentation fault" ==

Reason of kernel crash:

In md_bitmap_read_sb (called by md_bitmap_create), bad bitmap magic didn't
block chunksize assignment, and zero value made DIV_ROUND_UP_SECTOR_T()
trigger "divide error".

Crash log:

kernel: md: md0 stopped.
kernel: md/raid1:md0: not clean -- starting background reconstruction
kernel: md/raid1:md0: active with 2 out of 2 mirrors
kernel: dlm: ... ...
kernel: md-cluster: Joined cluster 44810aba-38bb-e6b8-daca-bc97a0b254aa slot 1
kernel: md0: invalid bitmap file superblock: bad magic
kernel: md_bitmap_copy_from_slot can't get bitmap from slot 2
kernel: md-cluster: Could not gather bitmaps from slot 2
kernel: divide error: 0000 [#1] SMP NOPTI
kernel: CPU: 0 PID: 1603 Comm: mdadm Not tainted 5.14.6-1-default
kernel: Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
kernel: RIP: 0010:md_bitmap_create+0x1d1/0x850 [md_mod]
kernel: RSP: 0018:ffffc22ac0843ba0 EFLAGS: 00010246
kernel: ... ...
kernel: Call Trace:
kernel: ? dlm_lock_sync+0xd0/0xd0 [md_cluster 77fe..7a0]
kernel: md_bitmap_copy_from_slot+0x2c/0x290 [md_mod 24ea..d3a]
kernel: load_bitmaps+0xec/0x210 [md_cluster 77fe..7a0]
kernel: md_bitmap_load+0x81/0x1e0 [md_mod 24ea..d3a]
kernel: do_md_run+0x30/0x100 [md_mod 24ea..d3a]
kernel: md_ioctl+0x1290/0x15a0 [md_mod 24ea....d3a]
kernel: ? mddev_unlock+0xaa/0x130 [md_mod 24ea..d3a]
kernel: ? blkdev_ioctl+0xb1/0x2b0
kernel: block_ioctl+0x3b/0x40
kernel: __x64_sys_ioctl+0x7f/0xb0
kernel: do_syscall_64+0x59/0x80
kernel: ? exit_to_user_mode_prepare+0x1ab/0x230
kernel: ? syscall_exit_to_user_mode+0x18/0x40
kernel: ? do_syscall_64+0x69/0x80
kernel: entry_SYSCALL_64_after_hwframe+0x44/0xae
kernel: RIP: 0033:0x7f4a15fa722b
kernel: ... ...
kernel: ---[ end trace 8afa7612f559c868 ]---
kernel: RIP: 0010:md_bitmap_create+0x1d1/0x850 [md_mod]