Re-enable CONFIG_MODVERSIONS in a slightly weaker form

This enables CONFIG_MODVERSIONS again, but allows for missing symbol CRC
information in order to work around the issue that newer binutils
versions seem to occasionally drop the CRC on the floor.  binutils 2.26
seems to work fine, while binutils 2.27 seems to break MODVERSIONS of
symbols that have been defined in assembler files.

[ We've had random missing CRC's before - it may be an old problem that
  just is now reliably triggered with the weak asm symbols and a new
  version of binutils ]

Some day I really do want to remove MODVERSIONS entirely.  Sadly, today
does not appear to be that day: Debian people apparently do want the
option to enable MODVERSIONS to make it easier to have external modules
across kernel versions, and this seems to be a fairly minimal fix for
the annoying problem.

Cc: Ben Hutchings <[email protected]>
Acked-by: Michal Marek <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

init/Kconfig

@@ -1945,7 +1945,6 @@ config MODULE_FORCE_UNLOAD
 
 config MODVERSIONS
 	bool "Module versioning support"
-	depends on BROKEN
 	help
 	  Usually, you have to use modules compiled with your kernel.
 	  Saying Y here makes it sometimes possible to use modules

kernel/module.c

@@ -1301,8 +1301,9 @@ static int check_version(Elf_Shdr *sechdrs,
 		goto bad_version;
 	}
 
-	pr_warn("%s: no symbol version for %s\n", mod->name, symname);
-	return 0;
+	/* Broken toolchain. Warn once, then let it go.. */
+	pr_warn_once("%s: no symbol version for %s\n", mod->name, symname);
+	return 1;
 
 bad_version:
 	pr_warn("%s: disagrees about version of symbol %s\n",