Fix unallocated object detection for large_dnode datasets

Fix dmu_object_next() to correctly handle unallocated objects on
large_dnode datasets.

We implement this by scanning the dnode block until we find the correct
offset to be used in dnode_next_offset(). This is necessary because we
can't assume *objectp is a hole even if dmu_object_info() returns
ENOENT.

This fixes a couple of issues with zfs receive on large_dnode datasets.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #5027
Closes #5532