x86/boot/e820: Clean up the E820_X_MAX definition

E820_X_MAX is defined in a somewhat messy fashion:

 - there's a pretty pointless looking #ifndef __KERNEL__ define that
   makes no sense in the non-UAPI header anymore,

 - part of it is defined in api.h, which is not for type definitions,

 - plus it's defined in two headers and the main explanation is in the
   header where we don't have the real definition.

So move it into a single place in e820/types.h and get rid of the
!__KERNEL__ case altogether. Drop the smaller comment - the larger
one explains it just fine.

Note that the zeropage does not use E820_X_MAX, it uses the legacy
128 entries definition.

No change in functionality.

Cc: Alex Thorlton <athorlton@sgi.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Huang, Ying <ying.huang@intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Jackson <pj@sgi.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>