osd/ReplicatedPG: redirect reads instead of promoting when full

osd/ReplicatedPG: redirect reads instead of promoting when full

If the cache pool is full, we are processing a read op, and we would
otherwise promote, redirect instead.  This lets us continue to process the
op without blocking or making the cache pool any more full than it is.

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