6529 Properly handle updates of variably-sized SA entries.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Ned Bass <bass6@llnl.gov>
Reviewed by: Tim Chase <tim@chase2k.com>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Andriy Gapon <avg@icyb.net.ua>

illumos/illumos-gate@e7e978b1f75353cb29673af9b35453c20c2827bf

During the update process in sa_modify_attrs(), the sizes of existing
variably-sized SA entries are obtained from sa_lengths[]. The case where
a variably-sized SA was being replaced neglected to increment the index
into sa_lengths[], so subsequent variable-length SAs would be rewritten
with the wrong length. This patch adds the missing increment operation
so all variably-sized SA entries are stored with their correct lengths.

Another problem was that index into attr_desc[] was increased even when
an attribute was removed. If that attribute was not the last attribute,
then the last attribute was lost.