pipewire: mem: pw_memblock_map(): fix pointer when reusing mapping

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

test/meson.build

@@ -52,6 +52,7 @@ test('test-pw-utils',
                'test-properties.c',
                'test-array.c',
                'test-map.c',
+               'test-mempool.c',
                'test-utils.c',
                include_directories: pwtest_inc,
                dependencies: [ spa_dep ],