kernel linker: Disable local sym resolution by default

In 95c20faf11 and ecd8245e0d kib introduced support to have the
kernel linker stop resolving local symbols from other files, but did
not enable it by default to avoid surprises.  Flip the default now,
before FreeBSD 16.0.

The debug.link_elf_leak_locals and debug.link_elf_obj_leak_locals
sysctls are available to revert to the previous behaviour if necessary.

PR:		207898
Reviewed by:	bz
Relnotes:	Yes
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47742

UPDATING

@@ -27,6 +27,15 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 16.x IS SLOW:
 	world, or to merely disable the most expensive debugging functionality
 	at runtime, run "ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
+20251125:
+	The `debug.link_elf_leak_locals` and `debug.link_elf_obj_leak_locals`
+	sysctls now default to 0, so the kernel module linker no longer
+	performs symbol resolution against local symbols from other modules.
+
+	If a kernel module now fails to load because of an unresolved symbol,
+	set these sysctls to 1 and file a bug report including details about
+	the affected module.
+
 20251115:
 	The FreeBSD-base repository is now defined in /etc/pkg/FreeBSD.conf,
 	disabled by default.  In -CURRENT and -STABLE this points at nightly

sys/kern/link_elf.c

@@ -203,7 +203,7 @@ static struct linker_class link_elf_class = {
 	link_elf_methods, sizeof(struct elf_file)
 };
 
-static bool link_elf_leak_locals = true;
+static bool link_elf_leak_locals = false;
 SYSCTL_BOOL(_debug, OID_AUTO, link_elf_leak_locals,
     CTLFLAG_RWTUN, &link_elf_leak_locals, 0,
     "Allow local symbols to participate in global module symbol resolution");

sys/kern/link_elf_obj.c

@@ -192,7 +192,7 @@ static struct linker_class link_elf_class = {
 	link_elf_methods, sizeof(struct elf_file)
 };
 
-static bool link_elf_obj_leak_locals = true;
+static bool link_elf_obj_leak_locals = false;
 SYSCTL_BOOL(_debug, OID_AUTO, link_elf_obj_leak_locals,
     CTLFLAG_RWTUN, &link_elf_obj_leak_locals, 0,
     "Allow local symbols to participate in global module symbol resolution");