Use g_handleattr() to reply to GEOM::candelete queries.

g_handleattr() fills out bp->bio_completed; otherwise, g_getattr()
returns an error in response to the query. This caused BIO_DELETE
support to not be propagated through stacked configurations, e.g.,
a gconcat of gmirror volumes would not handle BIO_DELETE even when
the gmirrors do. g_io_getattr() was not affected by the problem.

PR: 232676
Reported and tested by: noah.bergbauer@tum.de
MFC after: 1 week