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>

kernel/bpf/arena.c

@@ -37,7 +37,7 @@
  */
 
 /* number of bytes addressable by LDX/STX insn with 16-bit 'off' field */
-#define GUARD_SZ (1ull << sizeof(((struct bpf_insn *)0)->off) * 8)
+#define GUARD_SZ (1ull << sizeof_field(struct bpf_insn, off) * 8)
 #define KERN_VM_SZ ((1ull << 32) + GUARD_SZ)
 
 struct bpf_arena {