selftests/mm: skip hugevm test if kernel config file is not present

selftests/mm: skip hugevm test if kernel config file is not present

When running hugevm tests in a machine without kernel config present,
e.g., a VM running a kernel without CONFIG_IKCONFIG_PROC nor
/boot/config-*, skip hugevm tests, which reads kernel config to get page
table level information.

Link: https://lkml.kernel.org/r/20250516132938.356627-3-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Adam Sindelar <adam@wowsignal.io>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>