libceph: allocate the locator string with GFP_NOFAIL

calc_target() isn't supposed to fail with anything but POOL_DNE, in
which case we report that the pool doesn't exist and fail the request
with -ENOENT.  Doing this for -ENOMEM is at the very least confusing
and also harmful -- as the preceding requests complete, a short-lived
locator string allocation is likely to succeed after a wait.

(We used to call ceph_object_locator_to_pg() for a pi lookup.  In
theory that could fail with -ENOENT, hence the "ret != -ENOENT" warning
being removed.)

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>