md/raid10: fix null-ptr-deref in raid10_sync_request

hulk inclusion
category: bugfix
bugzilla: 188378, https://gitee.com/openeuler/kernel/issues/I6GGV7


CVE: NA

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init_resync() init mempool and set conf->have_replacemnt at the begaining
of sync, close_sync() free the mempool when sync is completed.

After commit 7e83ccbe ("md/raid10: Allow skipping recovery when clean
arrays are assembled"), recovery might skipped and init_resync() is called
but close_sync() is not. null-ptr-deref occurs as below:
  1) creat a array, wait for resync to complete, mddev->recovery_cp is set
     to MaxSector.
  2) recovery is woken and it is skipped. conf->have_replacement is set to
     0 in init_resync(). close_sync() not called.
  3) some io errors and rdev A is set to WantReplacement.
  4) a new device is added and set to A's replacement.
  5) recovery is woken, A have replacement, but conf->have_replacemnt is
     0. r10bio->dev[i].repl_bio will not be alloced and null-ptr-deref
     occurs.

Fix it by not init_resync() if recovery skipped.

Fixes: 7e83ccbe md/raid10: Allow skipping recovery when clean arrays are assembled")
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>