Back when we relied on the journal sequence number blacklist machinery
for consistency between btree and the journal, we needed to ensure a new
journal entry was written before any btree writes were done. But, this
had the side effect of consuming some space in the journal prior to
doing journal replay - which could lead to a very wedged filesystem,
since we don't yet have a way to grow the journal prior to going RW.

Fortunately, the journal sequence number blacklist machinery isn't
needed anymore, as btree node pointers now record the numer of sectors
currently written to that node - that code should all be ripped out.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>