nfp: add a mutex lock for the vNIC ctrl BAR

Soon we will try to write to the vNIC mailbox without RTNL held.
Add a new mutex to protect access to specific parts of the PCI
control BAR.

Move the mailbox size checking to the mailbox lock() helper, where
it can be more effective (happen prior to potential overwrite of
other data).

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>