ext4: Fix 64-bit block type problem on 32-bit platforms

The function ext4_mb_free_blocks() was using an "unsigned long" to
pass a block number; this will cause 64-bit block numbers to get
truncated on x86 and other 32-bit platforms.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>