efivarfs: always register filesystem

The efivar ops are typically registered at subsys init time so that
they are available when efivarfs is registered at module init time.

Other efivars implementations, such as Google SMI, exist and can
currently be built as modules which means that efivar may not be
available when efivarfs is initialised.

Move the efivar availability check from module init to when the
filesystem is mounted to allow late registration of efivars.
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

fs/efivarfs/super.c

@@ -194,6 +194,9 @@ static int efivarfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	struct dentry *root;
 	int err;
 
+	if (!efivar_is_available())
+		return -EOPNOTSUPP;
+
 	sb->s_maxbytes          = MAX_LFS_FILESIZE;
 	sb->s_blocksize         = PAGE_SIZE;
 	sb->s_blocksize_bits    = PAGE_SHIFT;
@@ -243,6 +246,9 @@ static void efivarfs_kill_sb(struct super_block *sb)
 {
 	kill_litter_super(sb);
 
+	if (!efivar_is_available())
+		return;
+
 	/* Remove all entries and destroy */
 	efivar_entry_iter(efivarfs_destroy, &efivarfs_list, NULL);
 }
@@ -256,9 +262,6 @@ static struct file_system_type efivarfs_type = {
 
 static __init int efivarfs_init(void)
 {
-	if (!efivar_is_available())
-		return -ENODEV;
-
 	return register_filesystem(&efivarfs_type);
 }