erofs: always use iget5_locked

As Christoph said [1] [2], "Just use the slightly
more complicated 32-bit version everywhere so that
you have a single actually tested code path.
And then remove this helper. "

[1] https://lore.kernel.org/r/20190829102426.GE20598@infradead.org/
[2] https://lore.kernel.org/r/20190902125320.GA16726@infradead.org/
Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-25-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>