x86/e820: Make e820_search_gap() static and remove unused variables

e820_search_gap() is just used locally now and the 'start_addr' and 'end_addr'
parameters are fixed values. Also, 'gapstart' is not checked in this function
anymore.

So make the function static and remove those unused variables.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: akataria@vmware.com
Link: http://lkml.kernel.org/r/1482676551-11411-1-git-send-email-richard.weiyang@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>