linux: Do not allow slab caches to be merged

From the kernel documentation:

> For reduced kernel memory fragmentation, slab caches can be
> merged when they share the same size and other characteristics.
> This carries a risk of kernel heap overflows being able to
> overwrite objects from merged caches (and more easily control
> cache layout), which makes such heap attacks easier to exploit
> by attackers. By keeping caches unmerged, these kinds of exploits
> can usually only damage objects in the same cache. [...]

Thus, it is more sane to leave slab merging disabled. KSPP and ClipOS
recommend this as well.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Acked-by: Michael Tremer <michael.tremer@ipfire.org>

config/kernel/kernel.config.aarch64-ipfire

@@ -247,7 +247,7 @@ CONFIG_VM_EVENT_COUNTERS=y
 # CONFIG_SLAB is not set
 CONFIG_SLUB=y
 # CONFIG_SLOB is not set
-CONFIG_SLAB_MERGE_DEFAULT=y
+# CONFIG_SLAB_MERGE_DEFAULT is not set
 CONFIG_SLAB_FREELIST_RANDOM=y
 CONFIG_SLAB_FREELIST_HARDENED=y
 CONFIG_SHUFFLE_PAGE_ALLOCATOR=y

config/kernel/kernel.config.armv6l-ipfire

@@ -250,7 +250,7 @@ CONFIG_VM_EVENT_COUNTERS=y
 # CONFIG_SLAB is not set
 CONFIG_SLUB=y
 # CONFIG_SLOB is not set
-CONFIG_SLAB_MERGE_DEFAULT=y
+# CONFIG_SLAB_MERGE_DEFAULT is not set
 CONFIG_SLAB_FREELIST_RANDOM=y
 CONFIG_SLAB_FREELIST_HARDENED=y
 CONFIG_SHUFFLE_PAGE_ALLOCATOR=y

config/kernel/kernel.config.riscv64-ipfire

@@ -230,7 +230,7 @@ CONFIG_VM_EVENT_COUNTERS=y
 # CONFIG_SLAB is not set
 CONFIG_SLUB=y
 # CONFIG_SLOB is not set
-CONFIG_SLAB_MERGE_DEFAULT=y
+# CONFIG_SLAB_MERGE_DEFAULT is not set
 CONFIG_SLAB_FREELIST_RANDOM=y
 CONFIG_SLAB_FREELIST_HARDENED=y
 CONFIG_SHUFFLE_PAGE_ALLOCATOR=y

config/kernel/kernel.config.x86_64-ipfire

@@ -270,7 +270,7 @@ CONFIG_VM_EVENT_COUNTERS=y
 # CONFIG_SLAB is not set
 CONFIG_SLUB=y
 # CONFIG_SLOB is not set
-CONFIG_SLAB_MERGE_DEFAULT=y
+# CONFIG_SLAB_MERGE_DEFAULT is not set
 CONFIG_SLAB_FREELIST_RANDOM=y
 CONFIG_SLAB_FREELIST_HARDENED=y
 CONFIG_SHUFFLE_PAGE_ALLOCATOR=y