NFS: call nfs_wb_all() only on regular files

It looks like nfs_setattr() and nfs_rename() also need to test whether the
target is a regular file before calling nfs_wb_all()...

It isn't technically needed since the version of nfs_wb_all() that exists
on 2.6.16 should be safe to call on non-regular files (it will be a no-op).
However it is a useful optimisation.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>