libceph: don't go through with the mapping if the PG is too wide

With EC overwrites maturing, the kernel client will be getting exposed
to potentially very wide EC pools.  While "min(pi->size, X)" works fine
when the cluster is stable and happy, truncating OSD sets interferes
with resend logic (ceph_is_new_interval(), etc).  Abort the mapping if
the pool is too wide, assigning the request to the homeless session.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>