Follow-up r295924: Only sync hash-based db files open for writing when closing.

This fixes a major performance regression when reading db files such as
the pw database during a 'pkg install'.

MFC after: 1 week
Tested by: bapt
Reviewed by: bapt
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D5868