Add some comments explaining the locking and pinning algorithm when
using split pte locks.  Also implement a minor optimisation of not
pinning the PTE when not using split pte locks.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Xen-devel <xen-devel@lists.xensource.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>