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