NFS: Fix a potential file corruption issue when writing

NFS: Fix a potential file corruption issue when writing

patch 5d47a35600270e7115061cb1320ee60ae9bcb6b8 in mainline.

If the inode is flagged as having an invalid mapping, then we can't rely on
the PageUptodate() flag. Ensure that we don't use the "anti-fragmentation"
write optimisation in nfs_updatepage(), since that will cause NFS to write
out areas of the page that are no longer guaranteed to be up to date.

A potential corruption could occur in the following scenario:

client 1			client 2
===============			===============
				fd=open("f",O_CREAT|O_WRONLY,0644);
				write(fd,"fubar\n",6);	// cache last page
				close(fd);
fd=open("f",O_WRONLY|O_APPEND);
write(fd,"foo\n",4);
close(fd);

				fd=open("f",O_WRONLY|O_APPEND);
				write(fd,"bar\n",4);
				close(fd);
-----
The bug may lead to the file "f" reading 'fubar\n\0\0\0\nbar\n' because
client 2 does not update the cached page after re-opening the file for
write. Instead it keeps it marked as PageUptodate() until someone calls
invalidate_inode_pages2() (typically by calling read()).

The bug was introduced by commit 44b11874ff583b6e766a05856b04f3c492c32b84
"NFS: Separate metadata and page cache revalidation mechanisms"

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>