selftests: mincore: fix tmpfs mincore test failure

When running mincore test cases, I encountered the following failures:

"
mincore_selftest.c:359:check_tmpfs_mmap:Expected ra_pages (511) == 0 (0)
mincore_selftest.c:360:check_tmpfs_mmap:Read-ahead pages found in memory
check_tmpfs_mmap: Test terminated by assertion
          FAIL  global.check_tmpfs_mmap
not ok 5 global.check_tmpfs_mmap
FAILED: 4 / 5 tests passed
"

The reason for the test case failure is that my system automatically enabled
tmpfs large folio allocation by adding the 'transparent_hugepage_tmpfs=always'
cmdline. However, the test case still expects the tmpfs mounted on /dev/shm to
allocate small folios, which leads to assertion failures when verifying readahead
pages.

As discussed with David, there's no reason to continue checking the readahead
logic for tmpfs. Drop it to fix this issue.

Link: https://lkml.kernel.org/r/9a00856cc6a8b4e46f4ab8b1af11ce5fc1a31851.1744025467.git.baolin.wang@linux.alibaba.com


Fixes: d635ccdb ("mm: shmem: add a kernel command line to change the default huge policy for tmpfs")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>