selftests/mm: suppress compiler error in liburing check

selftests/mm: suppress compiler error in liburing check

When building the mm selftests on a system without liburing development
headers, check_config.sh leaks a raw compiler error:

  /tmp/tmp.kIIOIqwe3n.c:2:10: fatal error: liburing.h: No such file or directory
      2 | #include <liburing.h>
        |          ^~~~~~~~~~~~

Since this is an expected failure during the configuration probe,
redirect the compiler output to /dev/null to hide it.

And the build system prints a clear warning when this occurs:

  Warning: missing liburing support. Some tests will be skipped.

Because the user is properly notified about the missing dependency, the
raw compiler error is redundant and only confuse users.

Additionally, update the Makefile to use $(Q) and $(call msg,...) for the
check_config.sh execution.  This aligns the probe with standard kbuild
output formatting, providing a clean "CHK" message instead of printing the
raw command during the build.

Link: https://lore.kernel.org/20260422080446.26020-3-wangli.ahau@gmail.com
Signed-off-by: Li Wang <wangli.ahau@gmail.com>
Tested-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>