ceph: allocate capsnap memory outside of ceph_queue_cap_snap()

This will reduce very possible but unnecessary frequently memory
allocate/free in this loop.

URL: https://tracker.ceph.com/issues/44100


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>