mm/mm_init.c: remove the useless dma_reserve

Now nobody calls set_dma_reserve() to set value for dma_reserve, remove
set_dma_reserve(), global variable dma_reserve and the codes using it.

Link: https://lkml.kernel.org/r/20240325145646.1044760-3-bhe@redhat.com


Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>