alloc_tag: fix page_ext_get/page_ext_put sequence during page splitting

pgalloc_tag_sub() might call page_ext_put() using a page different from
the one used in page_ext_get() call.  This does not pose an issue since
page_ext_put() ignores this parameter as long as it's non-NULL but
technically this is wrong.  Fix it by storing the original page used in
page_ext_get() and passing it to page_ext_put().

Link: https://lkml.kernel.org/r/20240711220457.1751071-3-surenb@google.com
Fixes: be25d1d4

 ("mm: create new codetag references during page splitting")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Sourav Panda <souravpanda@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>