md: Don't register sync_thread for reshape directly

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

md: Don't register sync_thread for reshape directly

Currently, if reshape is interrupted, then reassemble the array will
register sync_thread directly from pers->run(), in this case
'MD_RECOVERY_RUNNING' is set directly, however, there is no guarantee
that md_do_sync() will be executed, hence stop_sync_thread() will hang
because 'MD_RECOVERY_RUNNING' can't be cleared.

Last patch make sure that md_do_sync() will set MD_RECOVERY_DONE,
however, following hang can still be triggered by dm-raid test
shell/lvconvert-raid-reshape.sh occasionally:

[root@fedora ~]# cat /proc/1982/stack
[<0>] stop_sync_thread+0x1ab/0x270 [md_mod]
[<0>] md_frozen_sync_thread+0x5c/0xa0 [md_mod]
[<0>] raid_presuspend+0x1e/0x70 [dm_raid]
[<0>] dm_table_presuspend_targets+0x40/0xb0 [dm_mod]
[<0>] __dm_destroy+0x2a5/0x310 [dm_mod]
[<0>] dm_destroy+0x16/0x30 [dm_mod]
[<0>] dev_remove+0x165/0x290 [dm_mod]
[<0>] ctl_ioctl+0x4bb/0x7b0 [dm_mod]
[<0>] dm_ctl_ioctl+0x11/0x20 [dm_mod]
[<0>] vfs_ioctl+0x21/0x60
[<0>] __x64_sys_ioctl+0xb9/0xe0
[<0>] do_syscall_64+0xc6/0x230
[<0>] entry_SYSCALL_64_after_hwframe+0x6c/0x74

Meanwhile mddev->recovery is:
MD_RECOVERY_RUNNING |
MD_RECOVERY_INTR |
MD_RECOVERY_RESHAPE |
MD_RECOVERY_FROZEN

Fix this problem by remove the code to register sync_thread directly
from raid10 and raid5. And let md_check_recovery() to register
sync_thread.