cma: split reservation of fixed area into a helper function

cma: split reservation of fixed area into a helper function

Move the check that verifies that reservation of fixed area does not cross
HIGHMEM boundary and the actual memblock_resrve() call into a helper
function.

This makes code more readable and decouples logic related to
CONFIG_HIGHMEM from the core functionality of
__cma_declare_contiguous_nid().

Link: https://lkml.kernel.org/r/20250703184711.3485940-3-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: Oscar Salvador <osalvador@suse.de>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: Pratyush Yadav <ptyadav@amazon.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>