NFS: Fix an Oops in nfs_setattr()

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

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>