x86/boot/e820: Consolidate 'struct e820_entry *entry' local variable names

So the E820 code has a lot of cases of:

	struct e820_entry *ei;

... but the 'ei' name makes very little sense if you think about it, it's
not an abbreviation of anything obviously related to E820 table entries.

This results in weird looking lines such as:

               if (type && ei->type != type)

where you might have to double check what 'ei' really means, plus
weird looking secondary variable names, such as:

	u64 ei_end;

The 'ei' name was introduced in a single function over a decade ago, and
then mindlessly cargo-copied over into other functions - with usage growing
to over 60 uses altogether (!).

( My best guess is that it might have been originally meant as abbreviation
  of 'entry interval'. )

Anyway, rename these to the much more obvious:

	struct e820_entry *entry;

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>