selftests/mm: fix exclusive_cow test fork() handling

selftests/mm: fix exclusive_cow test fork() handling

The test ignores the return value of fork(), so both the parent and the
(newly created) child run the COW verification loops and then call
hmm_buffer_free() before returning into the kselftest harness, which
_exit()s each side.  This duplicated teardown sequence has been observed
to manifest as a SIGSEGV in the test child, e.g.:

  hmm-tests[360141]: segfault (11) at 0 nip 10006964 lr 1000ac3c code 1
  in hmm-tests[6964,10000000+30000]

Fix this by adopting the same fork()-then-wait pattern already used by the
nearby anon_write_child / anon_write_child_shared tests in this file: the
child performs the COW verification and then _exit(0)s so it does not run
the test teardown, while the parent independently verifies COW, waits for
the child, and only then frees the buffer.

Link: https://lore.kernel.org/20260611034102.1030738-4-aboorvad@linux.ibm.com
Fixes: b659baea75469 ("mm: selftests for exclusive device memory")
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Cc: Alex Sierra <alex.sierra@amd.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Sayali Patil <sayalip@linux.ibm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>