osd/ReplicatedPG: EBUSY on cache-evict when watchers are present

osd/ReplicatedPG: EBUSY on cache-evict when watchers are present

Linger operations will follow the object to the cache pool when the pool
overlay process is set.  If we evict the object, the object_info_t will
go away along with the watch state and confusing things will happen.
Prevent that from happening by returning EBUSY when you try to evict a
watched object.

Note that you *can* flush a watched object, and the dirty flag will be
cleared.  But you still can't evict it.

Signed-off-by: Sage Weil <sage@inktank.com>