NFS: Ensure nfs_wcc_update_inode always converts file size to loff_t

The nfs_wcc_update_inode() function omits logic to convert the type of
the NFS on-the-wire value of a file's size (__u64) to the type of file
size value stored in struct inode (loff_t, which is signed).

Everywhere else in the NFS client I checked already correctly converts the
file size type.

This effects only very large files.

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