SUNRPC: Remove an unneeded implicit type cast when calling rpc_depopulate()

The two arguments of rpc_depopulate() that pass in inode numbers should use
the same type as inode->i_ino: unsigned long.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>