module: don't unlink the module until we've removed all exposure.

Otherwise we get a race between unload and reload of the same module:
the new module doesn't see the old one in the list, but then fails because
it can't register over the still-extant entries in sysfs:

 [  103.981925] ------------[ cut here ]------------
 [  103.986902] WARNING: at fs/sysfs/dir.c:536 sysfs_add_one+0xab/0xd0()
 [  103.993606] Hardware name: CrownBay Platform
 [  103.998075] sysfs: cannot create duplicate filename '/module/pch_gbe'
 [  104.004784] Modules linked in: pch_gbe(+) [last unloaded: pch_gbe]
 [  104.011362] Pid: 3021, comm: modprobe Tainted: G        W    3.9.0-rc5+ #5
 [  104.018662] Call Trace:
 [  104.021286]  [<c103599d>] warn_slowpath_common+0x6d/0xa0
 [  104.026933]  [<c1168c8b>] ? sysfs_add_one+0xab/0xd0
 [  104.031986]  [<c1168c8b>] ? sysfs_add_one+0xab/0xd0
 [  104.037000]  [<c1035a4e>] warn_slowpath_fmt+0x2e/0x30
 [  104.042188]  [<c1168c8b>] sysfs_add_one+0xab/0xd0
 [  104.046982]  [<c1168dbe>] create_dir+0x5e/0xa0
 [  104.051633]  [<c1168e78>] sysfs_create_dir+0x78/0xd0
 [  104.056774]  [<c1262bc3>] kobject_add_internal+0x83/0x1f0
 [  104.062351]  [<c126daf6>] ? kvasprintf+0x46/0x60
 [  104.067231]  [<c1262ebd>] kobject_add_varg+0x2d/0x50
 [  104.072450]  [<c1262f07>] kobject_init_and_add+0x27/0x30
 [  104.078075]  [<c1089240>] mod_sysfs_setup+0x80/0x540
 [  104.083207]  [<c1260851>] ? module_bug_finalize+0x51/0xc0
 [  104.088720]  [<c108ab29>] load_module+0x1429/0x18b0

We can teardown sysfs first, then to be sure, put the state in
MODULE_STATE_UNFORMED so it's ignored while we deconstruct it.

Reported-by: Veaceslav Falico <vfalico@redhat.com>
Tested-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

kernel/module.c

@@ -1862,12 +1862,12 @@ static void free_module(struct module *mod)
 {
 	trace_module_free(mod);
 
-	/* Delete from various lists */
-	mutex_lock(&module_mutex);
-	stop_machine(__unlink_module, mod, NULL);
-	mutex_unlock(&module_mutex);
 	mod_sysfs_teardown(mod);
 
+	/* We leave it in list to prevent duplicate loads, but make sure
+	 * that noone uses it while it's being deconstructed. */
+	mod->state = MODULE_STATE_UNFORMED;
+
 	/* Remove dynamic debug info */
 	ddebug_remove_module(mod->name);
 
@@ -1880,6 +1880,11 @@ static void free_module(struct module *mod)
 	/* Free any allocated parameters. */
 	destroy_params(mod->kp, mod->num_kp);
 
+	/* Now we can delete it from the lists */
+	mutex_lock(&module_mutex);
+	stop_machine(__unlink_module, mod, NULL);
+	mutex_unlock(&module_mutex);
+
 	/* This may be NULL, but that's OK */
 	unset_module_init_ro_nx(mod);
 	module_free(mod, mod->module_init);