bpf: Switch bpf_map_{area_alloc,area_mmapable_alloc}() to u64 size

Given we recently extended the original bpf_map_area_alloc() helper in
commit fc970227 ("bpf: Add mmap() support for BPF_MAP_TYPE_ARRAY"),
we need to apply the same logic as in ff1c08e1 ("bpf: Change size
to u64 for bpf_map_{area_alloc, charge_init}()"). To avoid conflicts,
extend it for bpf-next.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>