rbd: remove snapshot existence validation code

RBD_DEV_FLAG_EXISTS check in rbd_queue_workfn() is racy and leads to
inconsistent behaviour.  If the object (or its snapshot) isn't there,
the OSD returns ENOENT.  A read submitted before the snapshot removal
notification is processed would be zero-filled and ended with status
OK, while future reads would be failed with IOERR.  It also doesn't
handle a case when an image that is mapped read-only is removed.

On top of this, because watch is no longer established for read-only
mappings, we no longer get notifications, so rbd_exists_validate() is
effectively dead code.  While failing requests rather than returning
zeros is a good thing, RBD_DEV_FLAG_EXISTS is not it.

Signed-off-by: Ilya Dryomov <[email protected]>
Reviewed-by: Jason Dillaman <[email protected]>
Reviewed-by: Dongsheng Yang <[email protected]>

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