Fix warnings found using UBSAN

Fix warnings found using UBSAN

Compiling with -fsanitize=undefined -fsanitize=address causes some
warnings of C code that has undefined behavior according to the C
standard bugs.  None of the warnings should cause e2fsprogs
malfunction given a sane compiler running on architectures that Linux
can support.  Still, it's better to clean up to code than not.

To fix up a complaint of a negative shift in hash function, update the
very dated hash we had been using for the revoke table with the
current generic hash used by the kernel.

Other fixes are fairly self-explanatory.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>