ext2: ext2_find_entry() return -ENOENT if no entry found

Almost all callers of ext2_find_entry() transform NULL return value to
-ENOENT, so just let ext2_find_entry() retuen -ENOENT instead of NULL
if no valid entry found, and also switch to check the return value of
ext2_inode_by_name() in ext2_lookup() and ext2_get_parent().

Link: https://lore.kernel.org/r/20200608034043.10451-2-yi.zhang@huawei.com
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jan Kara <jack@suse.cz>