md/raid1: Fix a warning message in remove_wb()

The WARN_ON() macro doesn't take an error message, it just takes a
condition.  I've changed this to use WARN(1, "...") instead.

Fixes: 3e148a32 ("md/raid1: fix potential data inconsistency issue with write behind device")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Song Liu <songliubraving@fb.com>