selftests/bpf: Run arena_mmap serially and test MAP_FIXED addresses

Each arena reserves 4 GiB of vmalloc space. The KASAN shadow of arenas from
parallel tests can exhaust the memory of a CI VM. Thus commit a14f9ba080c1
("selftests/bpf: Run arena tests serially") made arena_* and libarena*
serial. Do the same for arena_mmap, which the commit below added later.

Add four subtests for "bpf: Don't move a MAP_FIXED address in
arena_get_unmapped_area()". Test that mmap() with MAP_FIXED or
MAP_FIXED_NOREPLACE fails with EINVAL if the range crosses a 4 GiB
boundary. Test that mmap() with MAP_FIXED also fails for a range that wraps
and for a map_extra range that ends above TASK_SIZE. Today
arena_map_alloc() accepts such a map_extra. map_extra_last_pages also
passes if the kernel refuses the map, because no task can map this range.

The arch code gets a range before the arena code tests it for a crossing.
For the last two ranges LoongArch returns -EINVAL, and powerpc with the
hash MMU returns -ENOMEM. There the two subtests do not reach that test. On
the other architectures with arenas, the arena code returns -EINVAL or
__get_unmapped_area() returns -ENOMEM. Thus the two subtests accept both
errnos. All these errors occur before mmap() changes a mapping.

Two other changes of that patch have no subtest. With MAP_FIXED the arch
code gets len and not 2 * len. A 64-bit task on x86, arm64 or riscv does
not see this. LoongArch, s390 and powerpc with the hash MMU compare the end
of the range with different limits. In arena_mmap the unsigned ret changes
only the errno of map_extra_last_pages, which accepts both.

Fixes: 14d3e5bf9ffe ("selftests/bpf: Test the length and the address of an arena mmap()")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261102064231.2153879-3-alexei.starovoitov@gmail.com

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