When a page fault occurs on an address below the stack-vma, UML tries to
expand the stack.

On i386 and x86_64, the failing address is compared to the current userspace
stack pointer.  If the failing address is below "esp-32" resp.  "rsp-128",
stack expansion is not allowed, and a SIGSEGV is given to the user.

This patch makes UML behave like i386/x86_64.
Signed-off-by: Bodo Stroesser <bstroesser@fujitsu-siemens.com>
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>