The mapoffset should be using in the mmap call as the offset.
Mapping the whole memory before the offset and then ignoring the part
before it seems like it can work but it actually has some problems:
1. some drivers (v4l2) use the mapoffset to calculate the buffer to map
2. we waste resources for mapped but unused pages.
The problem with the mapoffset is that it needs to be page aligned and
that used to be a problem in the past. Nowadays PipeWire no longer set
the mapoffset for any of the memory that it allocates.
SPA_MEMBER is misleading, all we're doing here is pointer+offset and a
type-casting the result. Rename to SPA_PTROFF which is more expressive (and
has the same number of characters so we don't need to re-indent).
