x86: fix code paths used by update_mptable

Impact: fix crashes under Xen due to unrobust e820 code

find_e820_area_size() must return a properly distinguishable and
out-of-bounds value when it fails, and -1UL does not meet that
criteria on i386/PAE. Additionally, callers of the function must
check against that value.

early_reserve_e820() should be prepared for the region found to be
outside of the addressable range on 32-bits.

e820_update_range_map() should not blindly update e820, but should do
all it work on the map it got a pointer passed for (which in 50% of the
cases is &e820_saved). It must also not call e820_add_region(), as that
again acts on e820 unconditionally.

The issues were found when trying to make this option work in our Xen
kernel (i.e. where some of the silent assumptions made in the code
would not hold).

Signed-off-by: Jan Beulich <jbeulich@novell.com>
LKML-Reference: <49B9171B.76E4.0078.0@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>