NFS: Stop sillyname renames and unmounts from racing

Added an active/deactive mechanism to the nfs_server structure
allowing async operations to hold off umount until the
operations are done.

Signed-off-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

diff --cc fs/nfs/client.c
Simple merge

diff --cc fs/nfs/internal.h
Simple merge

diff --cc fs/nfs/super.c
Simple merge

diff --cc fs/nfs/unlink.c
Simple merge

diff --cc include/linux/nfs_fs_sb.h
Simple merge