NFS: Kill the "defined but not used" compile error on nommu machines

Bryan Wu reports that when compiling NFS on nommu machines he gets a
"defined but not used" error on nfs_file_mmap().

The easiest fix is simply to get rid of the special casing in NFS, and
just always call generic_file_mmap() to set up the file.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>