ceph: use ceph_create_snap_context()

Now that we have a library routine to create snap contexts, use it.

This is part of:
    http://tracker.ceph.com/issues/4857

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

fs/ceph/snap.c

@@ -332,10 +332,9 @@ static int build_snap_context(struct ceph_snap_realm *realm)
 	err = -ENOMEM;
 	if (num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
 		goto fail;
-	snapc = kzalloc(sizeof(*snapc) + num*sizeof(u64), GFP_NOFS);
+	snapc = ceph_create_snap_context(num, GFP_NOFS);
 	if (!snapc)
 		goto fail;
-	atomic_set(&snapc->nref, 1);
 
 	/* build (reverse sorted) snap vector */
 	num = 0;