x86/mm: Add an early PGT_validated exit in _get_page_type()

x86/mm: Add an early PGT_validated exit in _get_page_type()

This is a continuation of the cleanup and commenting in:
  9186e96b199e ("x86/pv: Clean up _get_page_type()")
  8cc5036bc385 ("x86/pv: Fix ABAC cmpxchg() race in _get_page_type()")

With the re-arranged and newly commented logic, it's now obvious that the
second half of _get_page_type() only has work to do for page validation.

Introduce an early exit for PGT_validated.  This makes the fastpath marginally
faster, and simplifies the subsequent logic as it no longer needs to exclude
the fully validated case.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>