sunrpc: destroy rpc_inode_cachep after unregister_filesystem

Better to unregister the file system before destroying the kmem_cache
cache of the inodes, so that the inodes are freed before we are trying
to destroy it. Otherwise, kmem_cache yells that some objects are live.

Signed-off-by: Dan Aloni <dan@kernelim.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>