sfc: manually allocate and free vadaptors

To be able to use MC_CMD_VADAPTOR_SET_MAC, vadaptors must be
manually allocated and freed as automatic vadaptors will disappear
when their reference_count reaches zero, which must happen before
the MAC address is changed.

Vadaptors are allocated and freed in the vswitching_probe/remove
functions for PFs and VFs, and this means that vadaptors are restored
correctly following an MC reboot or other reset when required.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>