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.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>