ext4: add explicit casts when masking cluster sizes

ext4: add explicit casts when masking cluster sizes

commit f5a44db5d2d677dfbf12deee461f85e9ec633961 upstream.

The missing casts can cause the high 64-bits of the physical blocks to
be lost.  Set up new macros which allows us to make sure the right
thing happen, even if at some point we end up supporting larger
logical block numbers.

Thanks to the Emese Revfy and the PaX security team for reporting this
issue.

Reported-by: PaX Team <pageexec@freemail.hu>
Reported-by: Emese Revfy <re.emese@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
[bwh: Backported to 3.2:
 - Adjust context
 - Drop inapplicable change to ext4_ext_rm_leaf()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>