btrfs: Fix rcu_dereference usage outside of read critical section

Patch 11ac3f1da5fd ("btrfs: log, when replace, is canceled by the user")
added a new btrfs_info call with a couple of btrfs_dev_name() args. This
is wrong since the latter require being called in rcu read side
critical section. Fix it by instead calling btrfs_info_in_rcu. This
fixes the following splat:

=============================
WARNING: suspicious RCU usage
4.16.0-rc2-nbor torvalds#463 Not tainted
-----------------------------
fs/btrfs/dev-replace.c:318 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by btrfs/5698:
 #0:  (&fs_info->dev_replace.lock_finishing_cancel_unmount){+.+.}, at: [<00000000942cb4ee>] btrfs_dev_replace_cancel+0xac/0x3f0

stack backtrace:
CPU: 2 PID: 5698 Comm: btrfs Not tainted 4.16.0-rc2-nbor torvalds#463
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
Call Trace:
 dump_stack+0x85/0xc9
 lockdep_rcu_suspicious+0x123/0x170
 btrfs_dev_name.part.1+0x6d/0x80
 btrfs_dev_replace_cancel+0x330/0x3f0
 btrfs_ioctl+0x2751/0x65b0
 ? debug_check_no_locks_freed+0x290/0x290
 ? trace_hardirqs_on_caller+0x400/0x570
 ? trace_hardirqs_on+0xd/0x10
 ? btrfs_ioctl_get_supported_features+0x30/0x30
 ? __handle_mm_fault+0x1aca/0x3230
 ? lock_downgrade+0x650/0x650
 ? trace_hardirqs_on+0xd/0x10
 ? mem_cgroup_commit_charge+0xc0/0xdd0
 ? _raw_spin_unlock+0x27/0x40
 ? __handle_mm_fault+0x1aca/0x3230
 ? lock_downgrade+0x650/0x650
 ? vm_insert_page+0x650/0x650
 ? __vma_link_rb+0x125/0x1d0
 do_vfs_ioctl+0x184/0xf00
 ? do_vfs_ioctl+0x184/0xf00
 ? lock_downgrade+0x650/0x650
 ? ioctl_preallocate+0x1a0/0x1a0
 ? up_read+0x1f/0x40
 ? __do_page_fault+0x5c6/0xb30
 ? SyS_brk+0x412/0x5f0
 ? mm_fault_error+0x2e0/0x2e0
 SyS_ioctl+0x41/0x70
 ? do_vfs_ioctl+0xf00/0xf00
 do_syscall_64+0x19d/0x5d0
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

Fixes: 11ac3f1da5fd ("btrfs: log, when replace, is canceled by the user")
Signed-off-by: Nikolay Borisov <[email protected]>

fs/btrfs/dev-replace.c

@@ -744,7 +744,7 @@ int btrfs_dev_replace_cancel(struct btrfs_fs_info *fs_info)
 	ret = btrfs_commit_transaction(trans);
 	WARN_ON(ret);
 
-	btrfs_info(fs_info, "dev_replace from %s (devid %llu) to %s canceled",
+	btrfs_info_in_rcu(fs_info, "dev_replace from %s (devid %llu) to %s cancelled",
 		   btrfs_dev_name(src_device), src_device->devid,
 		   btrfs_dev_name(tgt_device));