mm: arch: make in_gate_area take an mm_struct instead of a task_struct

Morally, the question of whether an address lies in a gate vma should be asked
with respect to an mm, not a particular task.  Moreover, dropping the dependency
on task_struct will help make existing and future operations on mm's more
flexible and convenient.

Signed-off-by: Stephen Wilson <wilsons@start.ca>
Reviewed-by: Michel Lespinasse <walken@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>