selftests/mm: free dynamically allocated PMD-sized buffers in split_huge_page_test

selftests/mm: free dynamically allocated PMD-sized buffers in split_huge_page_test

Dynamically allocated buffers of PMD size for file-backed THP operations
(file_buf1 and file_buf2) were not freed on the success path and some
failure paths.  Since the function is called repeatedly in a loop for each
split order, this can cause significant memory leaks.

On architectures with large PMD sizes, repeated leaks could exhaust system
memory and trigger the OOM killer during test execution.

Ensure all allocated buffers are freed to maintain stable repeated test
runs.

Link: https://lore.kernel.org/060c673b376bbeeed2b1fb1d48a825e846654191.1779296493.git.sayalip@linux.ibm.com
Fixes: 035a112e5fd5 ("selftests/mm: make file-backed THP split work by writing PMD size data")
Signed-off-by: Sayali Patil <sayalip@linux.ibm.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>