ceph: print r_direct_hash in hex in __choose_mds() dout

It's hard to read, especially when it is:

  ceph:  __choose_mds 00000000b7bc9c15 is_hash=1 (-271041095) mode 0

At the same time, switch to __func__ to get rid of the checkpatch
warning.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>