f2fs: UBSAN: set boolean value iostat_enable correctly

When setting /sys/fs/f2fs/<DEV>/iostat_enable with non-bool value, UBSAN
reports the following warning.

[ 7562.295484] ================================================================================
[ 7562.296531] UBSAN: Undefined behaviour in fs/f2fs/f2fs.h:2776:10
[ 7562.297651] load of value 64 is not a valid value for type '_Bool'
[ 7562.298642] CPU: 1 PID: 7487 Comm: dd Not tainted 4.20.0-rc4+ #79
[ 7562.298653] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[ 7562.298662] Call Trace:
[ 7562.298760]  dump_stack+0x46/0x5b
[ 7562.298811]  ubsan_epilogue+0x9/0x40
[ 7562.298830]  __ubsan_handle_load_invalid_value+0x72/0x90
[ 7562.298863]  f2fs_file_write_iter+0x29f/0x3f0
[ 7562.298905]  __vfs_write+0x115/0x160
[ 7562.298922]  vfs_write+0xa7/0x190
[ 7562.298934]  ksys_write+0x50/0xc0
[ 7562.298973]  do_syscall_64+0x4a/0xe0
[ 7562.298992]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 7562.299001] RIP: 0033:0x7fa45ec19c00
[ 7562.299004] Code: 73 01 c3 48 8b 0d 88 92 2c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 83 3d dd eb 2c 00 00 75 10 b8 01 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8 ce 8f 01 00 48 89 04 24
[ 7562.299044] RSP: 002b:00007ffca52b49e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[ 7562.299052] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fa45ec19c00
[ 7562.299059] RDX: 0000000000000400 RSI: 000000000093f000 RDI: 0000000000000001
[ 7562.299065] RBP: 000000000093f000 R08: 0000000000000004 R09: 0000000000000000
[ 7562.299071] R10: 00007ffca52b47b0 R11: 0000000000000246 R12: 0000000000000400
[ 7562.299077] R13: 000000000093f000 R14: 000000000093f400 R15: 0000000000000000
[ 7562.299091] ================================================================================

So, if iostat_enable is enabled, set its value as true.
Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/sysfs.c

@@ -280,11 +280,17 @@ static ssize_t __sbi_store(struct f2fs_attr *a,
 		return count;
 	}
 
-	*ui = t;
 
-	if (!strcmp(a->attr.name, "iostat_enable") && *ui == 0)
+	if (!strcmp(a->attr.name, "iostat_enable")) {
+		sbi->iostat_enable = !!t;
+		if (!sbi->iostat_enable)
 			f2fs_reset_iostat(sbi);
 		return count;
+	}
+
+	*ui = (unsigned int)t;
+
+	return count;
 }
 
 static ssize_t f2fs_sbi_store(struct f2fs_attr *a,