Right now we have these four types of PAT-disabled boot messages:

  x86/PAT: PAT support disabled.
  x86/PAT: PAT MSR is 0, disabled.
  x86/PAT: MTRRs disabled, skipping PAT initialization too.
  x86/PAT: PAT not supported by CPU.

The first message is ambiguous in that it doesn't signal that PAT is off
due to a boot option.

The second message doesn't really make it clear that this is the MSR value
during early bootup and it's the firmware environment that disabled PAT
support.

The fourth message doesn't really make it clear that we disable PAT support
because CONFIG_MTRR is off in the kernel.

Clarify, harmonize and fix the spelling in these user-visible messages:

  x86/PAT: PAT support disabled via boot option.
  x86/PAT: PAT support disabled by the firmware.
  x86/PAT: PAT support disabled because CONFIG_MTRR is disabled in the kernel.
  x86/PAT: PAT not supported by the CPU.

Also add a fifth message, in case PAT support is disabled at build time:

  x86/PAT: PAT support disabled because CONFIG_X86_PAT is disabled in the kernel.

Previously we'd just silently return from pat_init() without giving any indication
that PAT support is off.

Finally, clarify/extend some of the comments related to PAT initialization.
Signed-off-by: Ingo Molnar <mingo@kernel.org>