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>