x86: PAT: clarify is_linear_pfn_mapping() interface

Impact: Documentation only

Incremental patches to address the review comments from Nick Piggin
for v3 version of x86 PAT pfnmap changes patchset here

http://lkml.indiana.edu/hypermail/linux/kernel/0812.2/01330.html



This patch:

Clarify is_linear_pfn_mapping() and its usage.

It is used by x86 PAT code for performance reasons. Identifying pfnmap
as linear over entire vma helps speedup reserve and free of memtype
for the region.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>