ceph: fix inode reference leakage in ceph_get_snapdir()

In the Linux kernel, the following vulnerability has been resolved: ceph: fix inode reference leakage in ceph_get_snapdir() The ceph_get_inode() will search for or insert a new inode into the hash for the given vino, and return a reference to it. If new is non-NULL, its reference is consumed. We should release the reference when in error handing cases.