i40e: Protect access to VF control methods

A scenario has been found in which simultaneous
addition/removal and modification of VF's might cause
unstable behaviour, up to and including kernel panics.

Protect the methods that create/modify/destroy VF's
by locking them behind an atomically set bit in PF status
bitfield.

Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>