arm64: fix PAGE_OFFSET calc for flipped mm

Since kernel commit 14c127c957c1 ('arm64: mm: Flip kernel VA space'),
the memory layout on arm64 have changed, and kexec-tools can no longer
get the the right PAGE_OFFSET based on _text symbol.

Prior to that, the kimage (_text) lays above PAGE_END with this layout:
0               -> VA_START                 : Usespace
VA_START        -> VA_START + 256M          : BPF JIT, Modules
VA_START + 256M -> PAGE_OFFSET - (~GB misc) : Vmalloc (KERNEL _text HERE)
PAGE_OFFSET     -> ...                      : * Linear map *

And here we have:
VA_START    = -1UL << VA_BITS
PAGE_OFFSET = -1UL << (VA_BITS - 1)
_text < -1UL << (VA_BITS - 1)

Kernel image lays somewhere between VA_START and PAGE_OFFSET, so we just
calc VA_BITS by getting the highest unset bit of _text symbol address,
and shift one less bit of VA_BITS to get page offset. This works as long
as KASLR don't put kernel in a too high location (which is commented inline).

And after that commit, kernel layout have changed:
0               -> PAGE_OFFSET              : Userspace
PAGE_OFFSET     -> PAGE_END                 : * Linear map *
PAGE_END        -> PAGE_END + 128M          : bpf jit region
PAGE_END + 128M -> PAGE_END + 256MB         : modules
PAGE_END + 256M -> ...                      : vmalloc (KERNEL _text HERE)

Here we have:
PAGE_OFFSET = -1UL << VA_BITS
PAGE_END    = -1UL << (VA_BITS - 1)
_text > -1UL << (VA_BITS - 1)

Kernel image now lays above PAGE_END, so we have to shift one more bit to
get the VA_BITS, and shift the exact VA_BITS for PAGE_OFFSET.

We can simply check if "_text > -1UL << (VA_BITS - 1)" is true to judge
which layout is being used and shift the page offset occordingly.

Signed-off-by: Kairui Song <kasong@tencent.com>
(rebased and stripped by Pingfan )
Signed-off-by: Pingfan Liu <piliu@redhat.com>
Reviewed-by: Philipp Rudo <prudo@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>