Previously the pointer was determined as follows:
mm->this.ptr = SPA_PTROFF(m->ptr, range.start, void);
however, when `pw_map_range` is calculated, `pw_map_range::start` is the offset
from the beginning of the first page, starting at `pw_map_range::offset`.
This works correctly if `memblock_map()` runs because that will map the file
with expected offset, so using `range.start` is correct.
However, when a mapping is reused (i.e. `memblock_find_mapping()`) finds something,
then `range.start` is not necessarily correct. Consider the following example:
* page size is 10
* one memblock with size 20 (2 pages)
* the applications wants to mappings:
* (offset=5,size=10)
* (offset=15,size=5)
After the first request from the application, a `mapping` object is created
that covers the first two pages of the memblock: offset=0 and size=20. During
the second request, the calculated `pw_map_range` is as follows:
{ start = 5, offset = 10, size = 10 }
and the only previously created mapping is reused since (0 <= 5) and (10 <= 20). When
the pointer of the mapping is adjusted afterwards it will be incorrect since `m->ptr`
points to byte 0 on page 0 (instead of byte 0 on page 1 -- that is assumed). Thereforce
the two will unexpectedly overlap.
Fix that by using `offset - m->offset` when adjusting the mapping's pointer. Also move
the `range` variable into a smaller scope because it only makes sense there. And add
a test that check the above previously incorrect case.
Fixes: 2caf81c97c ("mem: improve memory handling")
Fixes#4884
