ReplicatedPG: test for missing head before find_object_context

ReplicatedPG: test for missing head before find_object_context

find_object_context doesn't return EAGAIN for a missing head.
I chose not to change that behavior since it might hide bugs
in the future.  All other callers check for missing on head
before calling into find_object_context because we potentially
need head or snapdir to map a snapid onto a clone.

Backport: emperor
Fixes: 6758
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>
(cherry picked from commit dd9d8b020286d5e3a69455023c3724a7b436d687)