NFS: add an endian notation

This function returns a big endian value.  The implementation in
fs/nfs/callback_proc.c is declared with "__be32" but the .h file uses
"unsigned" instead.  It makes sparse complain:

fs/nfs/callback_proc.c:232:8: error:
	symbol 'nfs4_callback_layoutrecall' redeclared with different
	type (originally declared at fs/nfs/callback.h:165) - different
	base types

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>