MINOR: buf: add b_getblk_ofs() that works relative to area and not head

MINOR: buf: add b_getblk_ofs() that works relative to area and not head

For some concurrently accessed buffers we can't rely on head/data etc,
but sometimes the access patterns guarantees that the buffer contents
are there. Let's implement a function to read contents from a fixed
offset, which never checks head nor data, only the area and its size.
It's the caller's job to get this offset.