Handle damaged blk_birth in dsl_deadlist_insert()

If a bit were cleared in `bp->blk_birth` such that the txg birth
was now lower than any other txg_birth in the deadlist, then there
will be no entry before this in the tree.

This should be impossible but regardless error handling code has
been added for this case.  By default this is left as a fatal case
and the blk_birth is logged.  However, setting `zfs_recover=1` will
cause the bp to be placed at the start of the deadlist even though
it contains an invalid blk_birth.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Closes openzfs#4086
Closes openzfs#4089