ext4: remove unused variable warning in parse_options()

Commit c33fbe8 ("ext4: Enable blocksize < pagesize for
dioread_nolock") removed the only user of 'sbi' outside of the ifdef,
so it caused a new warning:

fs/ext4/super.c:2068:23: warning: unused variable 'sbi' [-Wunused-variable]

Fixes: c33fbe8 ("ext4: Enable blocksize < pagesize for dioread_nolock")
Signed-off-by: Olof Johansson <olof@lixom.net>
Link: https://lore.kernel.org/r/20191111022523.34256-1-olof@lixom.net
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>

fs/ext4/super.c

@@ -2051,7 +2051,7 @@ static int parse_options(char *options, struct super_block *sb,
 			 unsigned int *journal_ioprio,
 			 int is_remount)
 {
-	struct ext4_sb_info *sbi = EXT4_SB(sb);
+	struct ext4_sb_info __maybe_unused *sbi = EXT4_SB(sb);
 	char *p, __maybe_unused *usr_qf_name, __maybe_unused *grp_qf_name;
 	substring_t args[MAX_OPT_ARGS];
 	int token;