[PATCH] uml: clean up address space limits code

I was looking at the code of the UML and more precisely at the functions
set_task_sizes_tt and set_task_sizes_skas.  I noticed that these 2 functions
take a paramater (arg) which is not used : the function is always called with
the value 0.

I suppose that this value might change in the future (or even can be
configured), so I added a constant in mem_user.h file.

Also, I rounded CONFIG_HOST_TASk_SIZE to a 4M.

Signed-off-by: Tyler <tyler@agat.net>
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>