fat (exportfs): move NFS support code

fat (exportfs): move NFS support code

Under memory pressure, the system may evict dentries from cache.  When the
FAT driver receives a NFS request involving an evicted dentry, it is
unable to reconnect it to the filesystem root.  This causes the request to
fail, often with ENOENT.

This is partially due to ineffectiveness of the current FAT NFS
implementation, and partially due to an unimplemented fh_to_parent method.
 The latter can cause file accesses to fail on shares exported with
subtree_check.

This patch set provides the FAT driver with the ability to
reconnect dentries.  NFS file handle generation and lookups are simplified
and made congruent with ext2.

Testing has involved a memory-starved virtual machine running 3.5-rc5 that
exports a ~2 GB vfat filesystem containing a kernel tree (~770 MB, ~40000
files, 9 levels).  Both 'cp -r' and 'ls -lR' operations were performed
from a client, some overlapping, some consecutive.  Exports with
'subtree_check' and 'no_subtree_check' have been tested.

Note that while this patch set improves FAT's NFS support, it does not
eliminate ESTALE errors completely.

The following should be considered for NFS clients who are sensitive to ESTALE:

* Mounting with lookupcache=none
  Unfortunately this can degrade performance severely, particularly for deep
  filesystems.

* Incorporating VFS patches to retry ESTALE failures on the client-side,
  such as https://lkml.org/lkml/2012/6/29/381

* Handling ESTALE errors in client application code

This patch:

Move NFS-related code into its own C file.  No functional changes.

Signed-off-by: Steven J. Magnani <steve@digidescorp.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>