mm: Fix PageHead when !CONFIG_PAGEFLAGS_EXTENDED

Unfortunately with !CONFIG_PAGEFLAGS_EXTENDED, (!PageHead) is false, and
(PageHead) is true, for tail pages.  If this is indeed the intended
behavior, which I doubt because it breaks cache cleaning on some ARM
systems, then the nomenclature is highly problematic.

This patch makes sure PageHead is only true for head pages and PageTail
is only true for tail pages, and neither is true for non-compound pages.

Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>
Allow vmexit profiling utility to run

Allow direct access to the cycle counter from VMs and just return
gracefully on an hvc #0 call.

Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>
KVM: ARM: Introduce more trace events

We would like to see some more detailed trace events for performance
profiling.

Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>
KVM: ARM: Measuring hack!!!

Warning: Not for human consumption.