Fix encryption hierarchy issues with zfs recv -d

Currently, the recv_fix_encryption_hierarchy() function accepts
'destsnap' as one of its parameters. Originally, this was intended
to be the top-level dataset of a receive (whether or not the
receive was recursive). Unfortunately, this parameter actually is
simply the input that is passed in from the command line. When
the user specifies 'zfs recv -d', this string is actually only the
name of the receiving pool since the rest of the name is derived
from the send stream. This causes the function to fail, leaving
some datasets with an invalid encryption hierarchy.

This patch resolves this problem by passing in the top_zfs variable
instead. In order to make this work, this patch also includes some
changes that ensure the value is always present when we need it.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #9273
Closes #9309