crimson/os/seastore: don't let a later rewrite reset force_rewrite_conflict

rewrite_logical_extent() sets the flag with a plain assignment:

    t.force_rewrite_conflict = (extents.size() > 1);

The flag marks remaps that produced multiple extents, and with them lba
insertions the no-conflict publish path cannot handle. But the trimmer
and the cleaner rewrite a batch of extents in one transaction (the
do_for_each loops in async_cleaner.cc), so each extent overwrites what
the previous one set: extent A remaps into three pieces and sets the
flag, extent B remaps into one and clears it again.
should_use_no_conflict_publish() then routes a transaction with
structural btree changes through the publish-to-prior path the comment
above this line rules out, and that path rebases concurrently built
transactions incompletely, leaving them with stale mappings.

Make the flag sticky with |=. It is only cleared on transaction reset,
which is the intended lifetime.

Introduced by 02e511c5d5d; the batch callers predate it, so the reset has been
reachable from the start.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>