covers: the 6.18 lane no longer carries the defrag NO_FLUSH patch

covers: the 6.18 lane no longer carries the defrag NO_FLUSH patch

The 2026-09-16 rebase onto v6.18.52 met Filipe's broader fix for the
defrag reservation deadlock (upstream ba02eab28041f, backported to 6.18
as a9701dd06e82c: the reservation moves up-front into defrag_one_range()
and defrag_one_locked_target() no longer reserves).  Our 6.18-only
"btrfs: defrag: don't flush on reservation while holding the cluster
locked" was dropped in its favour, and the probe-margin hold that used
to hook defrag_one_locked_target() now reaches defrag through the
up-front btrfs_delalloc_reserve_space() -- the commit message of "hold
the probe margin until the metadata reservation charges it" already
lists that path, so nothing there changes.  Only carry_one_range() (the
log-tail carry) keeps a defrag.c hook.

Update the README (one 6.18-only patch now, not two) and the cover
letter paragraph that said both upstream-fixed bugs were carried only on
the 6.18 backport.

Assisted-by: Claude:claude-fable-5-1