x86/kexec: simplify the logic of mem_region_callback()

The expression `mstart + resource_size(res) - 1` is actually equivalent to
`res->end`, simplify the logic of this function to improve readability.

Link: https://lkml.kernel.org/r/20231212150506.31711-1-ytcoode@gmail.com


Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>