NFS: The 'fattr_valid' field in struct nfs_server should be unsigned int

Fix up a static compiler warning:
"fs/nfs/nfs4proc.c:3882 _nfs4_server_capabilities() warn: was expecting
a 64 bit value instead of '(1 << 11)'"

The fix is to convert the fattr_valid field to match the type of the
'valid' field in struct nfs_fattr.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>