FreeBSD: Add const qualifier to members of struct opensolaris_utsname

These members have directly references to the global variables
exposed by the kernel. They are not going to be changed by this
kernel module.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Zhenlei Huang <zlei@FreeBSD.org>
Closes #16210

module/os/freebsd/spl/spl_misc.c

@@ -37,6 +37,9 @@
 #include <sys/zfs_context.h>
 
 static struct opensolaris_utsname hw_utsname = {
+	.sysname = ostype,
+	.nodename = prison0.pr_hostname,
+	.release = osrelease,
 	.machine = MACHINE
 };
 
@@ -49,10 +52,6 @@ utsname(void)
 static void
 opensolaris_utsname_init(void *arg)
 {
-
-	hw_utsname.sysname = ostype;
-	hw_utsname.nodename = prison0.pr_hostname;
-	hw_utsname.release = osrelease;
 	snprintf(hw_utsname.version, sizeof (hw_utsname.version),
 	    "%d", osreldate);
 }