x86/traps: improve hypervisor stack overflow detection

x86/traps: improve hypervisor stack overflow detection

A sample Gentoo compliation of Xen contains

    lea    -0x1058(%rsp),%rsp
    orq    $0x0,(%rsp)
    lea    0x1020(%rsp),%rsp

Whatever the reason for silly code like this, it fools the current stack
overflow detection logic in the #DF handler (which triggers reliably on the
'orq' instruction).

Update the overflow condition to declare an overflow if %esp is anywhere
within the guard page, rather than just within the upper 8th of the page.

Additionally, check %esp against the expected stack base in all builds.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>