nl80211: fix null pointer dereference

If the interface is not in MESH mode, the command 'iw wlanx mpath del'
will cause kernel panic.

The root cause is null pointer access in mpp_flush_by_proxy(), as the
pointer 'sdata->u.mesh.mpp_paths' is NULL for non MESH interface.

Unable to handle kernel NULL pointer dereference at virtual address 00000068
[...]
PC is at _raw_spin_lock_bh+0x20/0x5c
LR is at mesh_path_del+0x1c/0x17c [mac80211]
[...]
Process iw (pid: 4537, stack limit = 0xd83e0238)
[...]
[<c021211c>] (_raw_spin_lock_bh) from [<bf8c7648>] (mesh_path_del+0x1c/0x17c [mac80211])
[<bf8c7648>] (mesh_path_del [mac80211]) from [<bf6cdb7c>] (extack_doit+0x20/0x68 [compat])
[<bf6cdb7c>] (extack_doit [compat]) from [<c05c309c>] (genl_rcv_msg+0x274/0x30c)
[<c05c309c>] (genl_rcv_msg) from [<c05c25d8>] (netlink_rcv_skb+0x58/0xac)
[<c05c25d8>] (netlink_rcv_skb) from [<c05c2e14>] (genl_rcv+0x20/0x34)
[<c05c2e14>] (genl_rcv) from [<c05c1f90>] (netlink_unicast+0x11c/0x204)
[<c05c1f90>] (netlink_unicast) from [<c05c2420>] (netlink_sendmsg+0x30c/0x370)
[<c05c2420>] (netlink_sendmsg) from [<c05886d0>] (sock_sendmsg+0x70/0x84)
[<c05886d0>] (sock_sendmsg) from [<c0589f4c>] (___sys_sendmsg.part.3+0x188/0x228)
[<c0589f4c>] (___sys_sendmsg.part.3) from [<c058add4>] (__sys_sendmsg+0x4c/0x70)
[<c058add4>] (__sys_sendmsg) from [<c0208c80>] (ret_fast_syscall+0x0/0x44)
Code: e2822c02 e2822001 e5832004 f590f000 (e1902f9f)
---[ end trace bbd717600f8f884d ]---

Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Link: https://lore.kernel.org/r/1569485810-761-1-git-send-email-miaoqing@codeaurora.org


[trim useless data from commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>