gnttab: make the grant table lock a read-write lock

gnttab: make the grant table lock a read-write lock

In combination with the per-active entry locks, the grant table lock
can be made a read-write lock since the majority of cases only the
read lock is required. The grant table read lock protects against
changes to the table version or size (which are done with the write
lock held).

The write lock is also required when two active entries must be
acquired.

The double lock is still required when updating IOMMU page tables.

With the lock contention being only on the maptrack lock (unless IOMMU
updates are required), performance and scalability is improved.

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>