kexec_core: remove superfluous page offset handling in segment loading

During kexec_segment loading, when copying the content of the segment
(i.e. kexec_segment::kbuf or kexec_segment::buf) to its associated pages,
kimage_load_{cma,normal,crash}_segment handle the case where the physical
address of the segment is not page aligned, e.g. in
kimage_load_normal_segment:

page = kimage_alloc_page(image, GFP_HIGHUSER, maddr);
// ...
ptr = kmap_local_page(page);
// ...
ptr += maddr & ~PAGE_MASK;
mchunk = min_t(size_t, mbytes,
PAGE_SIZE - (maddr & ~PAGE_MASK));
// ^^^^ Non page-aligned segments handled here ^^^
// ...
if (image->file_mode)
memcpy(ptr, kbuf, uchunk);
else
result = copy_from_user(ptr, buf, uchunk);

(similar logic is present in kimage_load_{cma,crash}_segment).

This is actually not needed because, prior to their loading, all
kexec_segments first go through a vetting step in
`sanity_check_segment_list`, which rejects any segment that is not
page-aligned:

for (i = 0; i < nr_segments; i++) {
unsigned long mstart, mend;
mstart = image->segment[i].mem;
mend = mstart + image->segment[i].memsz;
// ...
if ((mstart & ~PAGE_MASK) || (mend & ~PAGE_MASK))
return -EADDRNOTAVAIL;
// ...
}

In case `sanity_check_segment_list` finds a non-page aligned the whole
kexec load is aborted and no segment is loaded.

This means that `kimage_load_{cma,normal,crash}_segment` never actually
have to handle non page-aligned segments and `(maddr & ~PAGE_MASK) == 0`
is always true no matter if the segment is coming from a file (i.e.
`kexec_file_load` syscall), from a user-space buffer (i.e. `kexec_load`
syscall) or created by the kernel through `kexec_add_buffer`. In the
latter case, `kexec_add_buffer` actually enforces the page alignment:

/* Ensure minimum alignment needed for segments. */
kbuf->memsz = ALIGN(kbuf->memsz, PAGE_SIZE);
kbuf->buf_align = max(kbuf->buf_align, PAGE_SIZE);

[jbouron@amazon.com: v3]
Link: https://lkml.kernel.org/r/20251024155009.39502-1-jbouron@amazon.com
Link: https://lkml.kernel.org/r/20250929160220.47616-1-jbouron@amazon.com
Signed-off-by: Justinien Bouron <jbouron@amazon.com>
Reviewed-by: Gunnar Kudrjavets <gunnarku@amazon.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Alexander Graf <graf@amazon.com>
Cc: Marcos Paulo de Souza <mpdesouza@suse.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>