flush_thread doesn't need to do a full page table walk in order to clear the
address space.  It knows what the end result needs to be, so it can call unmap
directly.

This results in a 10-20% speedup in an exec from bash.
Signed-off-by: NJeff Dike <jdike@linux.intel.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>