erofs: always use iget5_locked

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[email protected]/
[2] https://lore.kernel.org/r/20190902125320[email protected]/
Reported-by: Christoph Hellwig <[email protected]>
Signed-off-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>