rbd-mirror: generally skip replay/resync if remote image is not primary

rbd-mirror: generally skip replay/resync if remote image is not primary

Replay and resync should generally be skipped if the remote image is
not primary.

If this is not done for replay, snapshot-based mirroring can run into
a livelock if the primary image is demoted while a mirror snapshot is
being synced.  On the demote site, rbd-mirror would pick up the just
demoted image, grab the exclusive lock on it and idle waiting for a new
mirror snapshot to be created.  On the (still) non-primary site,
rbd-mirror would eventually finish syncing that mirror snapshot and
attempt to unlink from it on the demote site.  These attempts would
fail with EROFS due to exclusive lock being held in the "refuse proxied
maintenance operations" mode, blocking forward progress (syncing of the
demotion snapshot so that the non-primary image can be orderly promoted
to primary, etc).

If this is not done for resync, data loss can ensue as the just demoted
image would be immediately trashed, underneath the non-primary site that
is still syncing.

Currently this is done in PrepareReplayRequest only for journal-based
mirroring.  Note that it is conditional: if the local image is linked
to the remote image, proceeding is desirable.

Generalize this check, consolidate it with a related check in
PrepareRemoteImageRequest and move the result to BootstrapRequest to
cover both "local image does not exist" and "local image is unlinked"
cases for both modes.

Fixes: https://tracker.ceph.com/issues/54448
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>