Null pointer dereference

In the Linux kernel, the following vulnerability has been resolved:

net: netdevsim: don’t try to destroy PHC on VFs

PHC gets initialized in nsim_init_netdevsim(), which
is only called if (nsim_dev_port_is_pf()).

Create a counterpart of nsim_init_netdevsim() and
move the mock_phc_destroy() there.

This fixes a crash trying to destroy netdevsim with
VFs instantiated, as caught by running the devlink.sh test:

BUG: kernel NULL pointer dereference, address: 00000000000000b8
RIP: 0010:mock_phc_destroy+0xd/0x30
Call Trace:
 <TASK>
 nsim_destroy+0x4a/0x70 [netdevsim]
 __nsim_dev_port_del+0x47/0x70 [netdevsim]
 nsim_dev_reload_destroy+0x105/0x120 [netdevsim]
 nsim_drv_remove+0x2f/0xb0 [netdevsim]
 device_release_driver_internal+0x1a1/0x210
 bus_remove_device+0xd5/0x120
 device_del+0x159/0x490
 device_unregister+0x12/0x30
 del_device_store+0x11a/0x1a0 [netdevsim]
 kernfs_fop_write_iter+0x130/0x1d0
 vfs_write+0x30b/0x4b0
 ksys_write+0x69/0xf0
 do_syscall_64+0xcc/0x1e0
 entry_SYSCALL_64_after_hwframe+0x6f/0x77