kexec_file: skip checksum verification when safe

Checksum verification is needed
1. for crash kernels. In a crash, we can't be sure the kernel is
   intact.
2. if we're worried about relocating the kernel into a region used by
   some DMA that wasn't properly cancelled.

If KHO is enabled then relocations will happen to KHO scratch, which
is free from DMA regions.
If we used CMA to allocate segments then relocations are not going to
happen at all.
Therefore, we can safely disable checksum verification in both of those
cases.

Instead of adding a new variable to purgatory, just skip adding regions
and save the default value of SHA256 hash.

Saves ~250ms on my 4.0 GHz CPU. This is an important saving for the
live-update project.

Signed-off-by: Michal Clapinski <mclapinski@google.com>
Reviewed-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Baoquan He <baoquan.he@linux.dev>
Link: https://patch.msgid.link/20260602123311.1841746-1-mclapinski@google.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>