MFC r242734, 242815:

MFC r242734, 242815:

 - Implement BIO_FLUSH support around journal entries.  This will not 100%
   solve power loss problems with dishonest write caches.  However, it
   should improve the situation and force a full fsck when it is unable
   to resolve with the journal.
 - Resolve a case where the journal could wrap in an unsafe way causing
   us to prematurely lose journal entries in very specific scenarios.

 - Correct rev 242734, segments can sometimes get stuck.  Be a bit more
   defensive with segment state.

git-svn-id: svn://svn.freebsd.org/base/stable/9@245775 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f