ext2: propagate errors up to ext2_find_entry()'s callers

ext2: propagate errors up to ext2_find_entry()'s callers

The same to commit <36de928641ee4> (ext4: propagate errors up to
ext4_find_entry()'s callers') in ext4, also return error instead of NULL
pointer in case of some error happens in ext2_find_entry() (e.g. -ENOMEM
or -EIO). This could avoid a negative dentry cache entry installed even
it failed to read directory block due to IO error.

Link: https://lore.kernel.org/r/20200608034043.10451-1-yi.zhang@huawei.com
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>