ext4: initialize err_blk before calling __ext4_get_inode_loc

It is not guaranteed that __ext4_get_inode_loc will definitely set
err_blk pointer when it returns EIO. To avoid using uninitialized
variables, let's first set err_blk to 0.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20211201163421.2631661-1-harshads@google.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org