ceph: Don't add dirty inode to dirty list if caps is in migration

Add dirty inode to cap_dirty_migrating list instead, this can avoid
ceph_flush_dirty_caps() entering infinite loop.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 0685235ffd9dbdb9ccbda587f8a3c83ad1d5a921)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>