Revert "nfs: take extra reference to fl->fl_file when running a LOCKU operation"

This reverts commit db2efec0.

William reported that he was seeing instability with this patch, which
is likely due to the fact that it can cause the kernel to take a new
reference to a filp after the last reference has already been put.

Revert this patch for now, as we'll need to fix this in another way.

Cc: stable@vger.kernel.org
Reported-by: William Dauchy <william@gandi.net>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Reviewed-by: "J. Bruce Fields" <bfields@fieldses.org>
Tested-by: "J. Bruce Fields" <bfields@fieldses.org>