selftests/mm/cow: modify the incorrect checking parameters

selftests/mm/cow: modify the incorrect checking parameters

In run_with_memfd_hugetlb(), some error handle have passed incorrect
parameters.  It should be "smem", but it was mistakenly written as "mem".

Let's fix it.

[gehao@kylinos.cn: fix other errant sites, per Anshuman]
Link: https://lkml.kernel.org/r/20250113050908.93638-1-hao.ge@linux.dev
Link: https://lkml.kernel.org/r/20250113032858.63670-1-hao.ge@linux.dev
Fixes: f8664f3c4a08 ("selftests/vm: cow: basic COW tests for non-anonymous pages")
Signed-off-by: Hao Ge <gehao@kylinos.cn>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>