VFS: add lookup hint for network file systems

I'm trying to speeding up mkdir(2) for network file systems.  A typical
mkdir(2) calls two inode_operations: lookup and mkdir.  The lookup
operation would fail with ENOENT in common case.  I think it is unnecessary
because the subsequent mkdir operation can check it.  In case of creat(2),
lookup operation is called with the LOOKUP_CREATE flag, so individual
filesystem can omit real lookup.  e.g.  nfs_lookup().

Here is a sample patch which uses LOOKUP_CREATE and O_EXCL on mkdir,
symlink and mknod.  This uses the gadget for creat(2).

And here is the result of a benchmark on NFSv3.
  mkdir(2) 10,000 times:
    original  50.5 sec
    patched   29.0 sec

Signed-off-by: ASANO Masahiro <masano@tnes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
(cherry picked from fab7bf44449b29f9d5572a5dd8adcf7c91d5bf0f commit)