Use a real VMA to map the 32bit vsyscall page

This interacts better with Hugh's upcomming VMA walk optimization
Also removes some ugly special cases.

Code roughly modelled after the ppc64 vdso version from Ben Herrenschmidt.
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>