get_unmapped_area handles MAP_FIXED on x86_64

Handle MAP_FIXED in x86_64 arch_get_unmapped_area(), simple case, just return
the address as passed in

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>