m68k: Truncate base in do_div()

commit ea077b1b96e073eac5c3c5590529e964767fc5f7 upstream.

Explicitly truncate the second operand of do_div() to 32 bits to guard
against bogus code calling it with a 64-bit divisor.

[Thorsten]

After upgrading from 3.2 to 3.10, mounting a btrfs volume fails with:

btrfs: setting nodatacow, compression disabled
btrfs: enabling auto recovery
btrfs: disk space caching is enabled