selftest/mm: ksm_functional_tests: refactor mmap_and_merge_range()

In order to extend test_prctl_fork() and test_prctl_fork_exec() to make
sure that deduplication really happens, mmap_and_merge_range() needs to be
refactored.

Firstly, mmap_and_merge_range() will be called with no need to call enable
KSM by madvise or prctl.  So, switch the 'bool use_prctl' parameter to
enum ksm_merge_mode.

Secondly, mmap_and_merge_range() will be called in child process in the
two testcases, it isn't appropriate to call ksft_test_result_{fail, skip},
because the global variables ksft_{fail, skip} aren't consistent with the
parent process.  Thus, convert calls of ksft_test_result_{fail, skip} to
ksft_print_msg(), return differrent error according to the two cases, and
rename mmap_and_merge_range() to __mmap_and_merge_range().  For existing
callers, introduce new mmap_and_merge_range() to handle different return
values of __mmap_and_merge_range().

Link: https://lkml.kernel.org/r/20240328111010.1502191-3-tujinjiang@huawei.com
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Stefan Roesch <shr@devkernel.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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