ext4: fix potential uninitialized access to retval in kmmpd

if (!ext4_has_feature_mmp(sb)) then retval can be unitialized before
we jump to the wait_to_exit label.

Fixes: 61bb4a1c ("ext4: fix possible UAF when remounting r/o a mmp-protected file system")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Link: https://lore.kernel.org/r/20210713022728.2533770-1-yebin10@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mmp.c

@@ -138,7 +138,7 @@ static int kmmpd(void *data)
 	unsigned mmp_check_interval;
 	unsigned long last_update_time;
 	unsigned long diff;
-	int retval;
+	int retval = 0;
 
 	mmp_block = le64_to_cpu(es->s_mmp_block);
 	mmp = (struct mmp_struct *)(bh->b_data);