Avoid Undefined Behavior in ffs_clusteracct()

    Change the type of 'bit' variable from int to unsigned int and use unsigned
    values consistently.

    sys/ufs/ffs/ffs_subr.c:336:10, shift exponent -1 is negative

    Detected with Kernel Undefined Behavior Sanitizer.

    Reported by <Harry Pantazis>

Submitted by: Pedro Giffuni