NFS: Be more careful about mapping file permissions

When mapping a directory, we want the MAY_WRITE permissions to reflect
whether or not we have permission to modify, add and delete the directory
entries. MAY_EXEC must map to lookup permissions.

On the other hand, for files, we want MAY_WRITE to reflect a permission
to modify and extend the file.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>