mm: move free_reserved_area() to mm/memblock.c

mm: move free_reserved_area() to mm/memblock.c

free_reserved_area() is related to memblock as it frees reserved memory
back to the buddy allocator, similar to what memblock_free_late() does.

Move free_reserved_area() to mm/memblock.c to prepare for further
consolidation of the functions that free reserved memory.

No functional changes.

Link: https://patch.msgid.link/20260323074836.3653702-5-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>