Fix rollback of mounted filesystems as broken by Illumos #3875 patch.

The Illumos #3875 patch reverted a part of ZoL's 7b3e34b which added
special-case error handling for zfs_rezget().  The error handling dealt
with the case  in which an all-ones object number ended up being passed
to dnode_hold() and causing an EINVAL to be returned from zfs_rezget().