common/buffer: use homebrew BUF_OFFSETOF to replace offsetof()

common/buffer: use homebrew BUF_OFFSETOF to replace offsetof()

because buffer::raw is not a standard layout type. so `offsetof()`
does not always work. let's use a homebrew macro to calculate the
address of `bptr_storage`.

this change silences the warning like:

src/common/buffer.cc:2156:15: warning: ‘offsetof’ within non-standard-layout type ‘ceph::buffer::v15_2_0::raw’ is conditionally-supported [-Winvalid-offsetof]
 2156 |      offsetof(buffer::raw, bptr_storage));

Signed-off-by: Kefu Chai <kchai@redhat.com>