slabtop: T7657: Op-mode command show system memory cache does not work

Enable `CONFIG_SLUB_DEBUG` in `vyos_defconfig` to resolve slabtop errors
and improve memory cache diagnostics. Previously, running
the command failed with `slabtop: Unable to create slabinfo structure`.

Author: alexandr-san4ez

scripts/package-build/linux-kernel/arch/x86/configs/vyos_defconfig

@@ -6167,7 +6167,7 @@ CONFIG_HAVE_KCSAN_COMPILER=y
 #
 CONFIG_PAGE_EXTENSION=y
 # CONFIG_DEBUG_PAGEALLOC is not set
-# CONFIG_SLUB_DEBUG is not set
+CONFIG_SLUB_DEBUG=y
 # CONFIG_PAGE_OWNER is not set
 # CONFIG_PAGE_TABLE_CHECK is not set
 CONFIG_PAGE_POISONING=y