fs: prevent out-of-bounds array speculation when closing a file descriptor

commit 609d54441493c99f21c1823dfd66fa7f4c512ff4 upstream.

Google-Bug-Id: 114199369
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>