minix: bug widening a binary "not" operation

"chunk_size" is an unsigned int and "pos" is an unsigned long.  The
"& ~(chunk_size-1)" operation clears the high 32 bits unintentionally.

The ALIGN() macro does the correct thing.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>