bcachefs; Don't use btree write buffer until journal replay is finished

The keys being replayed by journal replay have to be synchronized with
updates by other threads that overwrite them. We rely on btree node
locks for synchronizing - but since btree write buffer updates take no
btree locks, that won't work.

Instead, simply disable using the btree write buffer until journal
replay is finished.

This fixes a rare backpointers error in the merge_torture_flakey test.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>