fuse: notify: don't move pages

[ Upstream commit 0d278362 ]

fuse_try_move_page() is not prepared for replacing pages that have already
been read.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>