bpf: Plumb get_unmapped_area() callback into bpf_map_ops

Subsequent patches introduce bpf_arena that imposes special alignment
requirements on address selection.

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20240307031228.42896-4-alexei.starovoitov@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>