x86: kexec_file: clean up prepare_elf64_headers()

Removing bufp variable in prepare_elf64_headers() makes the code simpler
and more understandable.

Link: http://lkml.kernel.org/r/20180306102303.9063-7-takahiro.akashi@linaro.org


Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Acked-by: Dave Young <dyoung@redhat.com>
Tested-by: Dave Young <dyoung@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>