Do not set BIO_DONE if the BIO specifies a completion handler.

biowait() will otherwise race with completions of such BIOs. In-tree code
only calls biowait() on BIOs that do not specify a handler, so this change
should not have any functional impact.

Reviewed by: mav
MFC after: 1 month
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D9070