vxlan: Do not assume RTNL is held in vxlan_fdb_info()

vxlan: Do not assume RTNL is held in vxlan_fdb_info()

vxlan_fdb_info() is not always called with RTNL held or from an RCU
read-side critical section. For example, in the following call path:

vxlan_cleanup()
  vxlan_fdb_destroy()
    vxlan_fdb_notify()
      __vxlan_fdb_notify()
        vxlan_fdb_info()

The use of rtnl_dereference() can therefore result in the following
splat [1].

Fix this by dereferencing the nexthop under RCU read-side critical
section.

[1]
[May24 22:56] =============================
[  +0.004676] WARNING: suspicious RCU usage
[  +0.004614] 5.7.0-rc5-custom-16219-g201392003491 #2772 Not tainted
[  +0.007116] -----------------------------
[  +0.004657] drivers/net/vxlan.c:276 suspicious rcu_dereference_check() usage!
[  +0.008164]
              other info that might help us debug this:

[  +0.009126]
              rcu_scheduler_active = 2, debug_locks = 1
[  +0.007504] 5 locks held by bash/6892:
[  +0.004392]  #0: ffff8881d47e3410 (&sig->cred_guard_mutex){+.+.}-{3:3}, at: __do_execve_file.isra.27+0x392/0x23c0
[  +0.011795]  #1: ffff8881d47e34b0 (&sig->exec_update_mutex){+.+.}-{3:3}, at: flush_old_exec+0x510/0x2030
[  +0.010947]  #2: ffff8881a141b0b0 (ptlock_ptr(page)#2){+.+.}-{2:2}, at: unmap_page_range+0x9c0/0x2590
[  +0.010585]  #3: ffff888230009d50 ((&vxlan->age_timer)){+.-.}-{0:0}, at: call_timer_fn+0xe8/0x800
[  +0.010192]  #4: ffff888183729bc8 (&vxlan->hash_lock[h]){+.-.}-{2:2}, at: vxlan_cleanup+0x133/0x4a0
[  +0.010382]
              stack backtrace:
[  +0.005103] CPU: 1 PID: 6892 Comm: bash Not tainted 5.7.0-rc5-custom-16219-g201392003491 #2772
[  +0.009675] Hardware name: Mellanox Technologies Ltd. MSN2100-CB2FO/SA001017, BIOS 5.6.5 06/07/2016
[  +0.010155] Call Trace:
[  +0.002775]  <IRQ>
[  +0.002313]  dump_stack+0xfd/0x178
[  +0.003895]  lockdep_rcu_suspicious+0x14a/0x153
[  +0.005157]  vxlan_fdb_info+0xe39/0x12a0
[  +0.004775]  __vxlan_fdb_notify+0xb8/0x160
[  +0.004672]  vxlan_fdb_notify+0x8e/0xe0
[  +0.004370]  vxlan_fdb_destroy+0x117/0x330
[  +0.004662]  vxlan_cleanup+0x1aa/0x4a0
[  +0.004329]  call_timer_fn+0x1c4/0x800
[  +0.004357]  run_timer_softirq+0x129d/0x17e0
[  +0.004762]  __do_softirq+0x24c/0xaef
[  +0.004232]  irq_exit+0x167/0x190
[  +0.003767]  smp_apic_timer_interrupt+0x1dd/0x6a0
[  +0.005340]  apic_timer_interrupt+0xf/0x20
[  +0.004620]  </IRQ>

Fixes: 7b74b4517f67 ("vxlan: ecmp support for mac fdb entries")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reported-by: Amit Cohen <amitc@mellanox.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>