Add functions and macros as used upstream.

[dweeezil]

Added highbit64() and howmany() which are used in recent upstream code.

[prakashsurya]

LGTM.

[prakashsurya]

I think we'll need a Signed-off-by: line too, but I'm sure that can be added when it lands.

[behlendorf]

Arguably the right thing to do here is map both highbit() and highbit64() to the kernels fls64() function. This will get us a generic versions of this function and an optimized version for a variety of arches.

[dweeezil]

@behlendorf I had initially wanted to use fls64() from the kernel but wasn't sure whether it was universally available among the various architectures. Looking a bit further, I see we've got a check in one of the m4 files for fls64 and define our own if it's not in the kernel so I guess we'll always have fls64 available in one form or another.

For the moment, however, I've fixed the suffix on the large constant.

[behlendorf]

I've merged this as is because that what we used for testing. This should still probably be changed to use fls64() at some point.