ext4: fix lost error code reporting in __ext4_fill_super()

When code was factored out of __ext4_fill_super() into
ext4_percpu_param_init() the error return was discarded.  This meant
that it was possible for __ext4_fill_super() to return zero,
indicating success, without the struct super getting completely filled
in, leading to a potential NULL pointer dereference.

Reported-by:  <syzbot+bbf0f9a213c94f283a5c@syzkaller.appspotmail.com>
Fixes: 1f79467c ("ext4: factor out ext4_percpu_param_init() ...")
Link: https://syzkaller.appspot.com/bug?id=6dac47d5e58af770c0055f680369586ec32e144c


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jason Yan <yanaijie@huawei.com>