lib: Add CONFIG_LIB_KMEM_CACHE for full kmem_cache support

Add a Kconfig option to control whether full kmem_cache_free() and
kmem_cache_destroy() implementations are provided in lib/linux_compat.c

Most boards do not need these functions, so they can use simple inline
stubs in slab.h. Subsystems like ext4 that require proper cache
management can select CONFIG_LIB_KMEM_CACHE.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

lib/Kconfig

@@ -396,6 +396,14 @@ config TPL_TINY_MEMSET
 config RBTREE
 	bool
 
+config LIB_KMEM_CACHE
+	bool "Enable full kmem_cache implementation"
+	help
+	  Provide a proper kmem_cache implementation in lib/linux_compat.c
+	  that tracks allocated objects. This is needed by subsystems like
+	  ext4 that require cache management. When disabled, simple inline
+	  stubs are used instead.
+
 config BITREVERSE
 	bool "Bit reverse library from Linux"
 

lib/Makefile

@@ -135,6 +135,7 @@ obj-$(CONFIG_$(PHASE_)OF_LIBFDT) += fdtdec.o fdtdec_common.o fdt_print.o
 obj-y += hang.o
 obj-y += linux_compat.o
 obj-y += linux_string.o
+obj-$(CONFIG_$(PHASE_)LIB_KMEM_CACHE) += kmem_cache.o
 obj-$(CONFIG_$(PHASE_)LMB) += lmb.o
 obj-y += membuf.o
 obj-$(CONFIG_REGEX) += slre.o

lib/kmem_cache.c

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * kmem_cache implementation for U-Boot
+ *
+ * Copyright 2025 Canonical Ltd
+ * Written by Simon Glass <simon.glass@canonical.com>
+ */
+
+#include <malloc.h>
+#include <linux/slab.h>
+
+void kmem_cache_free(struct kmem_cache *cachep, void *obj)
+{
+	free(obj);
+}
+
+void kmem_cache_destroy(struct kmem_cache *cachep)
+{
+	free(cachep);
+}