linux/spl/kmem_cache: undefine kmem_cache_alloc before defining it

When compiling a kernel with bcachefs and zfs, the two macros will collide, making it impossible
to have both filesystems.

It is sufficient to just undefine the macro before calling it.

On why this should be in ZFS rather than bcachefs, currently, bcachefs is not a in-tree filesystem, but,
it has a reasonably high chance of getting included soon.

This avoids the breakage in ZFS early, this patch may be distributed downstream in NixOS and is already used there.

include/os/linux/spl/sys/kmem_cache.h

@@ -201,6 +201,12 @@ extern uint64_t spl_kmem_cache_entry_size(kmem_cache_t *cache);
     spl_kmem_cache_create(name, size, align, ctor, dtor, rclm, priv, vmp, fl)
 #define	kmem_cache_set_move(skc, move)	spl_kmem_cache_set_move(skc, move)
 #define	kmem_cache_destroy(skc)		spl_kmem_cache_destroy(skc)
+/*
+ * This is necessary to be compatible with other kernel modules
+ * or in-tree filesystem that may define kmem_cache_alloc,
+ * like bcachefs does it now.
+ */
+#undef kmem_cache_alloc
 #define	kmem_cache_alloc(skc, flags)	spl_kmem_cache_alloc(skc, flags)
 #define	kmem_cache_free(skc, obj)	spl_kmem_cache_free(skc, obj)
 #define	kmem_cache_reap_now(skc)	spl_kmem_cache_reap_now(skc)