rbd: get snapshot context after exclusive lock is ensured to be held

Move capturing the snapshot context into the image request state
machine, after exclusive lock is ensured to be held for the duration of
dealing with the image request.  This is needed to ensure correctness
of fast-diff states (OBJECT_EXISTS vs OBJECT_EXISTS_CLEAN) and object
deltas computed based off of them.  Otherwise the object map that is
forked for the snapshot isn't guaranteed to accurately reflect the
contents of the snapshot when the snapshot is taken under I/O.  This
breaks differential backup and snapshot-based mirroring use cases with
fast-diff enabled: since some object deltas may be incomplete, the
destination image may get corrupted.

Cc: [email protected]
Link: https://tracker.ceph.com/issues/61472
Signed-off-by: Ilya Dryomov <[email protected]>
Reviewed-by: Dongsheng Yang <[email protected]>

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