mm/mm_init: rename init_reserved_page to init_deferred_page

mm/mm_init: rename init_reserved_page to init_deferred_page

When CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, init_reserved_page()
function performs initialization of a struct page that would have been
deferred normally.

Rename it to init_deferred_page() to better reflect what the function does.

Link: https://lkml.kernel.org/r/20250225083017.567649-3-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Frank van der Linden <fvdl@google.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Changyuan Lyu <changyuanl@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>