osd: do not assume head obc object exists when getting snapdir

For a list-snaps operation on the snapdir, do not assume that the obc for the
head means the object exists.  This fixes a race between a head deletion and
a list-snaps that wrongly returns ENOENT, triggered by the DiffItersateStress
test when thrashing OSDs.

Fixes: #5183
Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>