ext4: remove an unused variable warning with CONFIG_QUOTA=n

The 'enable_quota' variable is only used in an CONFIG_QUOTA.
With CONFIG_QUOTA=n, compiler causes a harmless warning:

fs/ext4/super.c: In function ‘ext4_remount’:
fs/ext4/super.c:5840:6: warning: variable ‘enable_quota’ set but not used
  [-Wunused-but-set-variable]
  int enable_quota = 0;
              ^~~~~

Move 'enable_quota' into the same #ifdef CONFIG_QUOTA block
to remove an unused variable warning.
Signed-off-by: Austin Kim <austindh.kim@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210824034929.GA13415@raspberrypiSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -5726,10 +5726,10 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	unsigned long old_sb_flags, vfs_flags;
 	struct ext4_mount_options old_opts;
-	int enable_quota = 0;
 	ext4_group_t g;
 	int err = 0;
 #ifdef CONFIG_QUOTA
+	int enable_quota = 0;
 	int i, j;
 	char *to_free[EXT4_MAXQUOTAS];
 #endif
@@ -5934,7 +5934,9 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 					err = -EROFS;
 					goto restore_opts;
 				}
+#ifdef CONFIG_QUOTA
 			enable_quota = 1;
+#endif
 		}
 	}