qcow2: do not clear the dirty bit when reopening a read-only node

qcow2_reopen_prepare() clears the dirty bit whenever the node is
reopened read-only, with an unguarded header write. A read-only node
can still be dirty, inherited from an earlier writable session, and it
holds no BLK_PERM_WRITE to resolve that. A read-only to read-only
reopen of a dirty image therefore fails outright:

  $ qemu-io -r -f qcow2 dirty.qcow2 <<< $'reopen -r\nquit'
  qemu-io: failed while preparing to reopen image 'dirty.qcow2'

Where the file node below is writable the write is not refused early,
and bdrv_co_write_req_prepare() aborts on its BLK_PERM_WRITE assertion
instead.

Clear it only for a node that is writable now, the predicate
qcow2_do_open() already uses for the repair. bdrv_is_writable() also
excludes an inactive node, whose header must not be touched either.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Hanna Reitz <hreitz@redhat.com>
CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Cc: qemu-stable@nongnu.org
Message-ID: <20260824133729.1141990-2-den@openvz.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>