md: fix a suspicious RCU usage warning

RCU protection was removed in the commit 2d32777d

 ("raid1: remove rcu
protection to access rdev from conf").

However, the code in fix_read_error does rcu_dereference outside
rcu_read_lock - this triggers the following warning. The warning is
triggered by a LVM2 test shell/integrity-caching.sh.

This commit removes rcu_dereference.

=============================
WARNING: suspicious RCU usage
6.7.0 #2 Not tainted
-----------------------------
drivers/md/raid1.c:2265 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
no locks held by mdX_raid1/1859.

stack backtrace:
CPU: 2 PID: 1859 Comm: mdX_raid1 Not tainted 6.7.0 #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0x60/0x70
 lockdep_rcu_suspicious+0x153/0x1b0
 raid1d+0x1732/0x1750 [raid1]
 ? lock_acquire+0x9f/0x270
 ? finish_wait+0x3d/0x80
 ? md_thread+0xf7/0x130 [md_mod]
 ? lock_release+0xaa/0x230
 ? md_register_thread+0xd0/0xd0 [md_mod]
 md_thread+0xa0/0x130 [md_mod]
 ? housekeeping_test_cpu+0x30/0x30
 kthread+0xdc/0x110
 ? kthread_complete_and_exit+0x20/0x20
 ret_from_fork+0x28/0x40
 ? kthread_complete_and_exit+0x20/0x20
 ret_from_fork_asm+0x11/0x20
 </TASK>

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: ca294b34

 ("md/raid1: support read error check")
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/51539879-e1ca-fde3-b8b4-8934ddedcbc@redhat.com