cred: crextend(): Harden, simplify

[olce]

Summary

Harden by adding more assertions, and a plain panic in case of an
unrepresentable size for the groups array (this can never happen after
the change of the 'kern.ngroups' computation to impose some not too high
maximum value a few commits ago). Fix an impact in kern_setgroups().

Simplify by removing the iterative process whose purpose is actually to
determine the closest power of two that is greater than the wanted
number of bytes. Using the proper target quantity (number of bytes)
incidentally helps with eliminating divisions (and the reliance on
sizeof(gid_t) being a power of two).

[mhorne]

sys/kern/kern_prot.c

2368

formally it should be size_t.

[olce]

sys/kern/kern_prot.c

2368

Oh, you're right. The int comes from an initial version where the variable still held a number of groups as the original code, and I forgot to change the type when I switched to a number of bytes. Which makes me notice that an overflow check is missing (as in the original code), I'll add one after the nbytes computation, coupled with another change in D46913.

[olce]

Switch nbytes to size_t, check for overflow with a plain panic (following D46913 latest change).

[olce]

Amend the comment in crcopy(), and add one in sys/cred.h to request to move the cr_ngroups field out of the copied area the next time a change of struct ucred's ABI is necessary.