FreeBSD: Implement sysctl for fletcher4 impl

There is a tunable to select the fletcher 4 checksum implementation on
Linux but it was not present in FreeBSD.

Implement the sysctl handler for FreeBSD and use ZFS_MODULE_PARAM_CALL
to provide the tunable on both platforms.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11270

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

@@ -57,6 +57,8 @@
 #define	ZFS_MODULE_PARAM_CALL(scope_prefix, name_prefix, name, func, _, perm, desc) \
     ZFS_MODULE_PARAM_CALL_IMPL(_vfs_ ## scope_prefix, name, perm, func ## _args(name_prefix ## name), desc)
 
+#define	ZFS_MODULE_VIRTUAL_PARAM_CALL ZFS_MODULE_PARAM_CALL
+
 #define	param_set_arc_long_args(var) \
     CTLTYPE_ULONG, &var, 0, param_set_arc_long, "LU"
 
@@ -84,6 +86,9 @@
 #define	param_set_max_auto_ashift_args(var) \
     CTLTYPE_U64, &var, 0, param_set_max_auto_ashift, "QU"
 
+#define	fletcher_4_param_set_args(var) \
+    CTLTYPE_STRING, NULL, 0, fletcher_4_param, "A"
+
 #include <sys/kernel.h>
 #define	module_init(fn)							\
 static void \