igb: revert rtnl_lock() that causes deadlock

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

igb: revert rtnl_lock() that causes deadlock

The commit 6faee3d4ee8b ("igb: Add lock to avoid data race") adds
rtnl_lock to eliminate a false data race shown below

(FREE from device detaching) | (USE from netdev core)
igb_remove | igb_ndo_get_vf_config
igb_disable_sriov | vf >= adapter->vfs_allocated_count?
kfree(adapter->vf_data) |
adapter->vfs_allocated_count = 0 |
| memcpy(... adapter->vf_data[vf]

The above race will never happen and the extra rtnl_lock causes deadlock
below

[ 141.420169] <TASK>
[ 141.420672] __schedule+0x2dd/0x840
[ 141.421427] schedule+0x50/0xc0
[ 141.422041] schedule_preempt_disabled+0x11/0x20
[ 141.422678] __mutex_lock.isra.13+0x431/0x6b0
[ 141.423324] unregister_netdev+0xe/0x20
[ 141.423578] igbvf_remove+0x45/0xe0 [igbvf]
[ 141.423791] pci_device_remove+0x36/0xb0
[ 141.423990] device_release_driver_internal+0xc1/0x160
[ 141.424270] pci_stop_bus_device+0x6d/0x90
[ 141.424507] pci_stop_and_remove_bus_device+0xe/0x20
[ 141.424789] pci_iov_remove_virtfn+0xba/0x120
[ 141.425452] sriov_disable+0x2f/0xf0
[ 141.425679] igb_disable_sriov+0x4e/0x100 [igb]
[ 141.426353] igb_remove+0xa0/0x130 [igb]
[ 141.426599] pci_device_remove+0x36/0xb0
[ 141.426796] device_release_driver_internal+0xc1/0x160
[ 141.427060] driver_detach+0x44/0x90
[ 141.427253] bus_remove_driver+0x55/0xe0
[ 141.427477] pci_unregister_driver+0x2a/0xa0
[ 141.428296] __x64_sys_delete_module+0x141/0x2b0
[ 141.429126] ? mntput_no_expire+0x4a/0x240
[ 141.429363] ? syscall_trace_enter.isra.19+0x126/0x1a0
[ 141.429653] do_syscall_64+0x5b/0x80
[ 141.429847] ? exit_to_user_mode_prepare+0x14d/0x1c0
[ 141.430109] ? syscall_exit_to_user_mode+0x12/0x30
[ 141.430849] ? do_syscall_64+0x67/0x80
[ 141.431083] ? syscall_exit_to_user_mode_prepare+0x183/0x1b0
[ 141.431770] ? syscall_exit_to_user_mode+0x12/0x30
[ 141.432482] ? do_syscall_64+0x67/0x80
[ 141.432714] ? exc_page_fault+0x64/0x140
[ 141.432911] entry_SYSCALL_64_after_hwframe+0x72/0xdc

Since the igb_disable_sriov() will call pci_disable_sriov() before
releasing any resources, the netdev core will synchronize the cleanup to
avoid any races. This patch removes the useless rtnl_(un)lock to guarantee
correctness.