[PATCH] btrfs: fix NPD during canceling replace when the target is missing

[Anand Jain <anand.jain@oracle.com>]

Replace target can be missing after a reboot during the replace.
So check if device is null.

BUG: unable to handle kernel NULL pointer dereference at 00000000000000b0
IP: btrfs_destroy_dev_replace_tgtdev+0x43/0xf0 [btrfs]
Call Trace:
btrfs_dev_replace_cancel+0x22b/0x250 [btrfs]
btrfs_ioctl+0x2216/0x2590 [btrfs]
? do_vfs_ioctl+0x625/0x650
do_vfs_ioctl+0x625/0x650
? security_file_ioctl+0x30/0x50
SyS_ioctl+0x4e/0x80
do_syscall_64+0x5d/0x160
entry_SYSCALL64_slow_path+0x25/0x25

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 fs/btrfs/dev-replace.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
index 87f975143c05..476981c2cf55 100644
--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -749,7 +749,8 @@ int btrfs_dev_replace_cancel(struct btrfs_fs_info *fs_info)
 		   btrfs_dev_name(src_device), src_device->devid,
 		   btrfs_dev_name(tgt_device));
 
-	btrfs_destroy_dev_replace_tgtdev(fs_info, tgt_device);
+	if (tgt_device)
+		btrfs_destroy_dev_replace_tgtdev(fs_info, tgt_device);
 
 leave:
 	mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
-- 
2.15.0