bpf: Use bpf_map_area_alloc consistently on bpf map creation

Let's use the generic helper bpf_map_area_alloc() instead of the
open-coded kzalloc helpers in bpf maps creation path.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20220810151840.16394-5-laoar.shao@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>