selftests/mm/uffd: remove static address usage in shmem_allocate_area()

selftests/mm/uffd: remove static address usage in shmem_allocate_area()

The current shmem_allocate_area() implementation uses a hardcoded virtual
base address (BASE_PMD_ADDR) as a hint for mmap() when creating
shmem-backed test areas.  This approach is fragile and may fail on systems
with ASLR or different virtual memory layouts, where the chosen address is
unavailable.

Replace the static base address with a dynamically reserved address range
obtained via mmap(NULL, ..., PROT_NONE).  The memfd-backed areas and their
alias are then mapped into that reserved region using MAP_FIXED,
preserving the original layout and aliasing semantics while avoiding
collisions with unrelated mappings.

This change improves robustness and portability of the test suite without
altering its behavior or coverage.

[mehdi.benhadjkhelifa@gmail.com: make cleanup code more clear, per Mike]
Link: https://lkml.kernel.org/r/20251113142050.108638-1-mehdi.benhadjkhelifa@gmail.com
Link: https://lkml.kernel.org/r/20251111205739.420009-1-mehdi.benhadjkhelifa@gmail.com
Signed-off-by: Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@gmail.com>
Suggested-by: Mike Rapoport <rppt@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Hunter <david.hunter.linux@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>