ext4: Fix ext4_mb_init_cache return error

ext4_mb_init_cache() incorrectly always return EIO on success. This
causes the caller of ext4_mb_init_cache() fail when it checks the return
value.

Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>