kexec_core: fix the assignment to kimage->control_page

kexec_core: fix the assignment to kimage->control_page

image->control_page represents the starting address for allocating the
next control page, while hole_end represents the address of the last valid
byte of the currently allocated control page.

This bug actually does not affect the correctness of allocating control
pages, because image->control_page is currently only used in
kimage_alloc_crash_control_pages(), and this function, when allocating
control pages, will first align image->control_page up to the nearest
`(1 << order) << PAGE_SHIFT` boundary, then use this value as the
starting address of the next control page.  This ensures that the newly
allocated control page will use the correct starting address and not
overlap with previously allocated control pages.

Although it does not affect the correctness of the final result, it is
better for us to set image->control_page to the correct value, in case
it might be used elsewhere in the future, potentially causing errors.

Therefore, after successfully allocating a control page,
image->control_page should be updated to `hole_end + 1`, rather than
hole_end.

Link: https://lkml.kernel.org/r/20231221042308.11076-1-ytcoode@gmail.com
Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>