FreeBSD: make adjustments for the standalone environment

In FreeBSD, there are three compile environments that are supported:
user land, the kernel and the bootloader / standalone. Adjust the
headers to compile in the standalone environment. Limit kernel-only
items from view when _STANDALONE is defined.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Warner Losh <imp@FreeBSD.org>
Closes #10998

include/os/freebsd/spl/sys/kmem.h

@@ -29,6 +29,7 @@
 #ifndef _OPENSOLARIS_SYS_KMEM_H_
 #define	_OPENSOLARIS_SYS_KMEM_H_
 
+#ifdef _KERNEL
 #include <sys/param.h>
 #include <sys/malloc.h>
 #include <sys/vmem.h>
@@ -93,5 +94,14 @@ void *calloc(size_t n, size_t s);
 	zfs_kmem_alloc((size), (kmflags) | M_ZERO)
 #define	kmem_free(buf, size)		zfs_kmem_free((buf), (size))
 
+#endif	/* _KERNEL */
+
+#ifdef _STANDALONE
+/*
+ * At the moment, we just need it for the type. We redirect the alloc/free
+ * routines to the usual Free and Malloc in that environment.
+ */
+typedef int kmem_cache_t;
+#endif /* _STANDALONE */
 
 #endif	/* _OPENSOLARIS_SYS_KMEM_H_ */