NFS: null dereference in dev_remove()

In commit 5ffaf855 "NFS: replace global bl_wq with per-net one" we
made "msg" a pointer instead of a struct stored in stack memory.  But we
forgot to change the memset() here so we're still clearing stack memory
instead clearing the struct like we intended.  It will lead to a kernel
crash.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>