Fix incorrect assertion in dnode_dirty_l1range

Fix incorrect assertion in dnode_dirty_l1range

The db_dirtycnt of an EVICTING dbuf is always 0. However, it still
appears in the dn_dbufs tree. If we call dnode_dirty_l1range on a
range that contains an EVICTING dbuf, we will attempt to mark it dirty
(which will fail because it's EVICTING, resulting in a new dbuf being
created and dirtied). Later, in ZFS_DEBUG mode, we assert that all the
dbufs in the range are dirty. If the EVICTING dbuf is still present,
this will trip the assertion erroneously.

Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Sara Hartse <sara.hartse@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #8745