nfs: Fix oops due to derefence ->lookup method

During mount if rpc_ops->getroot() and rpc_ops->getattr() returns different filehandles then nfs_getfh() set i_op of NFS root inode to nfs_mountpoint_inode_operations.

After that nfs_follow_remote_path() raised oops:

     d_alloc_and_lookup+0x4c/0x74
     do_lookup+0x1e3/0x280
     link_path_walk+0x12e/0xab0
     nfs4_remote_get_sb+0x56/0x2c0 [nfs]
     path_walk+0x67/0xe0
     vfs_path_lookup+0x8e/0x100
     nfs_follow_remote_path+0x16f/0x3e0 [nfs]
     nfs4_try_mount+0x6f/0xd0 [nfs]
     nfs_get_sb+0x269/0x400 [nfs]
     vfs_kern_mount+0x8a/0x1f0
     do_kern_mount+0x52/0x130
     do_mount+0x20a/0x260
     sys_mount+0x90/0xe0
     system_call_fastpath+0x16/0x1b

So just refresh fsid, as RFC3530 doesn't specify behavior in case of rootfh changes fsid.

---
 fs/nfs/getroot.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

index 3031ef7..914eb78 100644
@@ -192,6 +192,10 @@ struct dentry *nfs4_get_root(struct super_block

*sb, struct nfs_fh *mntfh)
goto out;
}

+ if (fattr->valid & NFS_ATTR_FATTR_FSID &&
+ !nfs_fsid_equal(&server->fsid, &fattr->fsid))
+ memcpy(&server->fsid, &fattr->fsid, sizeof(server->fsid));
+
inode = nfs_fhget(sb, mntfh, fattr);
if (IS_ERR(inode)) {
dprintk("nfs_get_root: get root inode failed\n");