selftests/mm: cow: print ksft header before printing anything else

Doing a ksft_print_msg() before the ksft_print_header() seems to confuse
the ksft framework in a strange way: running the test on the cmdline
results in the expected output.

But piping the output somewhere else, results in some odd output,
whereby we repeatedly get the same info printed:
	# [INFO] detected THP size: 2048 KiB
	# [INFO] detected hugetlb page size: 2048 KiB
	# [INFO] detected hugetlb page size: 1048576 KiB
	# [INFO] huge zeropage is enabled
	TAP version 13
	1..190
	# [INFO] Anonymous memory tests in private mappings
	# [RUN] Basic COW after fork() ... with base page
	# [INFO] detected THP size: 2048 KiB
	# [INFO] detected hugetlb page size: 2048 KiB
	# [INFO] detected hugetlb page size: 1048576 KiB
	# [INFO] huge zeropage is enabled
	TAP version 13
	1..190
	# [INFO] Anonymous memory tests in private mappings
	# [RUN] Basic COW after fork() ... with base page
	ok 1 No leak from parent into child
	# [RUN] Basic COW after fork() ... with swapped out base page
	# [INFO] detected THP size: 2048 KiB
	# [INFO] detected hugetlb page size: 2048 KiB
	# [INFO] detected hugetlb page size: 1048576 KiB
	# [INFO] huge zeropage is enabled

Doing the ksft_print_header() first seems to resolve that and gives us
the output we expect:
	TAP version 13
	# [INFO] detected THP size: 2048 KiB
	# [INFO] detected hugetlb page size: 2048 KiB
	# [INFO] detected hugetlb page size: 1048576 KiB
	# [INFO] huge zeropage is enabled
	1..190
	# [INFO] Anonymous memory tests in private mappings
	# [RUN] Basic COW after fork() ... with base page
	ok 1 No leak from parent into child
	# [RUN] Basic COW after fork() ... with swapped out base page
	ok 2 No leak from parent into child
	# [RUN] Basic COW after fork() ... with THP
	ok 3 No leak from parent into child
	# [RUN] Basic COW after fork() ... with swapped-out THP
	ok 4 No leak from parent into child
	# [RUN] Basic COW after fork() ... with PTE-mapped THP
	ok 5 No leak from parent into child

Link: https://lkml.kernel.org/r/20231206103558.38040-1-david@redhat.com


Fixes: f4b5fd69 ("selftests/vm: anon_cow: THP tests")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reported-by: Nico Pache <npache@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>