bpf,arena: Use helper sizeof_field in struct accessors

Use the well defined helper sizeof_field() to calculate the size of a
struct member, instead of doing custom calculations.

Signed-off-by: Haiyue Wang <haiyue.wang@intel.com>
Link: https://lore.kernel.org/r/20240327065334.8140-1-haiyue.wang@intel.com


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