ceph: all allocation functions should get gfp_mask

This is essential, as for the rados block device we'll need
to run in different contexts that would need flags that
are other than GFP_NOFS.

Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>