gnttab: per-active entry locking

gnttab: per-active entry locking

Introduce a per-active entry spin lock to protect active entry state
The grant table lock must be locked before acquiring (locking) an
active entry.

This is a step in reducing contention on the grant table lock, but
will only do so once the grant table lock is turned into a read-write
lock.

Based on a patch originally by Matt Wilson <msw@amazon.com>.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>