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

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

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

init_resync() inits mempool and sets conf->have_replacemnt at the beginning
of sync, close_sync() frees the mempool when sync is completed.

After [1] recovery might be skipped and init_resync() is called but
close_sync() is not. null-ptr-deref occurs with r10bio->dev[i].repl_bio.

The following is one way to reproduce the issue.

1) create 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 calling init_resync() if recovery skipped.

[1] commit 7e83ccbecd60 ("md/raid10: Allow skipping recovery when clean arrays are assembled")