x86/boot: Simplify EBDA-vs-BIOS reservation logic

Both the intent and the effect of reserve_bios_regions() is simple:
reserve the range from the apparent BIOS start (suitably filtered)
through 1MB and, if the EBDA start address is sensible, extend that
reservation downward to cover the EBDA as well.

The code is overcomplicated, though, and contains head-scratchers
like:

	if (ebda_start < BIOS_START_MIN)
		ebda_start = BIOS_START_MAX;

That snipped is trying to say "if ebda_start < BIOS_START_MIN,
ignore it".

Simplify it: reorder the code so that it makes sense.  This should
have no functional effect under any circumstances.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Mario Limonciello <mario_limonciello@dell.com>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Toshi Kani <toshi.kani@hp.com>
Link: http://lkml.kernel.org/r/ef89c0c761be20ead8bd9a3275743e6259b6092a.1469135598.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>