add the mutex lock for create/delete/config/insert nat46 devices to fix nat46 module crash issues.

[junnanx]

The nat46 module crash when get create/delete/config/insert same time.
The nat46 module can revices the applition by /proc/net/nat46/control file at the same time. So it need add the mutex lock for create/delete/config/insert nat46 devices.

[   95.827044] <0>.(0)[1485:464xlatcfg]nat46: deleting device (464-xlatltewan)
[   95.827251] <1>.(1)[2230:464xlatcfg]nat46: adding device (464-xlatltewan)
[   95.827956] <0>.(0)[1485:464xlatcfg]Destroying '464-xlatltewan'
[   95.828809] <1>.(1)[2230:464xlatcfg]Can not add: device '464-xlatltewan' already exists!
[   95.829555] <0>.(0)[1485:464xlatcfg][nat46] release_nat46_instance: freeing nat46 instance with 1 pairs
[   95.830577] <1>.(1)[2230:464xlatcfg]nat46: configure device (464-xlatltewan) with 'local.style NONE local.v4 192.0.0.1/32 local.v6 2001:50:55:a6:b896:3138:e9b1:ac3c/128 remote.style RFC6052 remote.v6 2002:172:903::c000:aa/96'
[   95.834325] <1>.(1)[2230:464xlatcfg][name:fault&]Unable to handle kernel NULL pointer dereference at virtual address 000000000000000c

[   96.982806] <1>-(1)[2230:464xlatcfg]Hardware name: MediaTek evb6890v1_64_cpe (DT)
[   96.983752] <1>-(1)[2230:464xlatcfg]Call trace:
[   96.984332] <1>-(1)[2230:464xlatcfg] dump_backtrace+0x0/0x150
[   96.985061] <1>-(1)[2230:464xlatcfg] show_stack+0x24/0x30
[   96.985748] <1>-(1)[2230:464xlatcfg] dump_stack+0x90/0xb8
[   96.986435] <1>-(1)[2230:464xlatcfg] mrdump_common_die+0x2a0/0x2b0
[   96.987218] <1>-(1)[2230:464xlatcfg] ipanic_die+0x38/0x44
[   96.987904] <1>-(1)[2230:464xlatcfg] notifier_call_chain+0x70/0x90
[   96.988688] <1>-(1)[2230:464xlatcfg] atomic_notifier_call_chain+0x3c/0x4c
[   96.989547] <1>-(1)[2230:464xlatcfg] notify_die+0x28/0x30
[   96.990233] <1>-(1)[2230:464xlatcfg] die+0xf4/0x294
[   96.990853] <1>-(1)[2230:464xlatcfg] die_kernel_fault+0xa0/0xc8
[   96.991604] <1>-(1)[2230:464xlatcfg] __do_kernel_fault+0xd8/0x100
[   96.992376] <1>-(1)[2230:464xlatcfg] do_page_fault+0x3dc/0x3fc
[   96.993116] <1>-(1)[2230:464xlatcfg] do_translation_fault+0x54/0x98
[   96.993911] <1>-(1)[2230:464xlatcfg] do_mem_abort+0x6c/0x14c
[   96.994629] <1>-(1)[2230:464xlatcfg] el1_da+0x1c/0xac
[   96.995272] <1>-(1)[2230:464xlatcfg] nat46_set_config+0x24/0x5c [nat46]
[   96.996111] <1>-(1)[2230:464xlatcfg] nat46_configure+0x44/0x58 [nat46]
[   96.996940] <1>-(1)[2230:464xlatcfg] nat46_destroy_all+0x340/0x3e0 [nat46]
[   96.997812] <1>-(1)[2230:464xlatcfg] proc_reg_write+0x7c/0xbc
[   96.998542] <1>-(1)[2230:464xlatcfg] do_loop_readv_writev+0x7c/0xcc
[   96.999337] <1>-(1)[2230:464xlatcfg] do_iter_write+0x100/0x11c
[   97.000077] <1>-(1)[2230:464xlatcfg] vfs_writev+0x94/0xd8
[   97.000762] <1>-(1)[2230:464xlatcfg] do_writev+0x68/0xc0
[   97.001437] <1>-(1)[2230:464xlatcfg] __arm64_sys_writev+0x28/0x34
[   97.002211] <1>-(1)[2230:464xlatcfg] el0_svc_common.constprop.0+0x94/0x114
[   97.003082] <1>-(1)[2230:464xlatcfg] el0_svc_handler+0x44/0x78
[   97.003821] <1>-(1)[2230:464xlatcfg] el0_svc+0x8/0xc