md/raid5: avoid BUG_ON() while continue reshape after reassembling

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

md/raid5: avoid BUG_ON() while continue reshape after reassembling

Currently, mdadm support --revert-reshape to abort the reshape while

reassembling, as the test 07revert-grow. However, following BUG_ON()

can be triggerred by the test:

kernel BUG at drivers/md/raid5.c:6278!

invalid opcode: 0000 [#1] PREEMPT SMP PTI

irq event stamp: 158985

CPU: 6 PID: 891 Comm: md0_reshape Not tainted 6.9.0-03335-g7592a0b0049a #94

RIP: 0010:reshape_request+0x3f1/0xe60

Call Trace:

<TASK>

raid5_sync_request+0x43d/0x550

md_do_sync+0xb7a/0x2110

md_thread+0x294/0x2b0

kthread+0x147/0x1c0

ret_from_fork+0x59/0x70

ret_from_fork_asm+0x1a/0x30

</TASK>

Root cause is that --revert-reshape update the raid_disks from 5 to 4,

while reshape position is still set, and after reassembling the array,

reshape position will be read from super block, then during reshape the

checking of 'writepos' that is caculated by old reshape position will

fail.

Fix this panic the easy way first, by converting the BUG_ON() to

WARN_ON(), and stop the reshape if checkings fail.

Noted that mdadm must fix --revert-shape as well, and probably md/raid

should enhance metadata validation as well, however this means

reassemble will fail and there must be user tools to fix the wrong

metadata.