selftests: skip mincore.check_file_mmap when fs lacks needed support

Report mincore.check_file_mmap as SKIP instead of FAIL if the underlying
filesystem lacks support of O_TMPFILE or fallocate since such failures
are not really related to mincore functionality.

Cc: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>