selftests/mincore: Allow read-ahead pages to reach the end of the file

When running the mincore_selftest on a system with an XFS file system, it
failed the "check_file_mmap" test case due to the read-ahead pages reaching
the end of the file. The failure log is as below:

   RUN           global.check_file_mmap ...
  mincore_selftest.c:264:check_file_mmap:Expected i (1024) < vec_size (1024)
  mincore_selftest.c:265:check_file_mmap:Read-ahead pages reached the end of the file
  check_file_mmap: Test failed
           FAIL  global.check_file_mmap

This is because the read-ahead window size of the XFS file system on this
machine is 4 MB, which is larger than the size from the #PF address to the
end of the file. As a result, all the pages for this file are populated.

  blockdev --getra /dev/nvme0n1p5
    8192
  blockdev --getbsz /dev/nvme0n1p5
    512

This issue can be fixed by extending the current FILE_SIZE 4MB to a larger
number, but it will still fail if the read-ahead window size of the file
system is larger enough. Additionally, in the real world, read-ahead pages
reaching the end of the file can happen and is an expected behavior.
Therefore, allowing read-ahead pages to reach the end of the file is a
better choice for the "check_file_mmap" test case.

Link: https://lore.kernel.org/r/20250311080940.21413-1-qiuxu.zhuo@intel.com


Reported-by: Yi Lai <yi1.lai@intel.com>
Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>