NFS: Fix an Oops when truncating a file

NFS: Fix an Oops when truncating a file

The VM/VFS does not allow mapping->a_ops->invalidatepage() to fail.
Unfortunately, nfs_wb_page_cancel() may fail if a fatal signal occurs.
Since the NFS code assumes that the page stays mapped for as long as the
writeback is active, we can end up Oopsing (among other things).

The only safe fix here is to convert nfs_wait_on_request(), so as to make
it uninterruptible (as is already the case with wait_on_page_writeback()).

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@kernel.org