Force --enable-debug on FreeBSD if INVARIANTS is set

There's already logic to force INVARIANTS on for building if it's
present in the running kernel; however, not having DEBUG enabled
when DEBUG and INVARIANTS are can cause strange panics.

Closes: openzfs#12163

Signed-off-by: Rich Ercolani <rincebrain@gmail.com>

config/zfs-build.m4

@@ -49,6 +49,11 @@ AC_DEFUN([ZFS_AC_DEBUG], [
 		[ZFS_AC_DEBUG_DISABLE],
 		[AC_MSG_ERROR([Unknown option $enable_debug])])
 
+	AS_CASE(["x$FORCE_DEBUG"],
+		["xtrue"],
+		[ZFS_AC_DEBUG_ENABLE],
+		["x"],[],[])
+
 	AC_SUBST(DEBUG_CFLAGS)
 	AC_SUBST(DEBUG_CPPFLAGS)
 	AC_SUBST(DEBUG_LDFLAGS)
@@ -173,6 +178,8 @@ dnl # in the FreeBSD kernel module.  This feature must be used when building
 dnl # for a FreeBSD kernel with "options INVARIANTS" in the KERNCONF and must
 dnl # not be used when the INVARIANTS option is absent.
 dnl #
+dnl # (If INVARIANTS is detected, we need to force DEBUG, or strange panics
+dnl # can ensue.)
 AC_DEFUN([ZFS_AC_DEBUG_INVARIANTS], [
 	AC_MSG_CHECKING([whether FreeBSD kernel INVARIANTS checks are enabled])
 	AC_ARG_ENABLE([invariants],
@@ -181,9 +188,12 @@ AC_DEFUN([ZFS_AC_DEBUG_INVARIANTS], [
 		[], [ZFS_AC_DEBUG_INVARIANTS_DETECT])
 
 	AS_IF([test "x$enable_invariants" = xyes],
-		[WITH_INVARIANTS="true"],
-		[WITH_INVARIANTS=""])
+		[WITH_INVARIANTS="true"
+		 FORCE_DEBUG="true"],
+		[WITH_INVARIANTS=""
+		 FORCE_DEBUG=""])
 	AC_SUBST(WITH_INVARIANTS)
+	AC_SUBST(FORCE_DEBUG)
 
 	AC_MSG_RESULT([$enable_invariants])
 ])

configure.ac

@@ -58,11 +58,11 @@ _AM_PROG_TAR(pax)
 ZFS_AC_LICENSE
 ZFS_AC_CONFIG
 ZFS_AC_PACKAGE
+ZFS_AC_DEBUG_INVARIANTS
 ZFS_AC_DEBUG
 ZFS_AC_DEBUGINFO
 ZFS_AC_DEBUG_KMEM
 ZFS_AC_DEBUG_KMEM_TRACKING
-ZFS_AC_DEBUG_INVARIANTS
 
 AC_CONFIG_FILES([
 	Makefile