copy_from_user_*() and buffer zeroing

["H. Peter Anvin" <hpa@zytor.com>]

I just started looking into the horribly confused state of buffer
zeroing for the various copy_from_user variants.  This came up after we
did some minor tuning last week.

copy_from_user_inatomic() seems to be documented to not zero the buffer.
 This is definitely *NOT* true on x86-64, although it does seem to be
true on i386 -- on x86-64, we carry along a "zerorest" flag but in all
possible codepaths it will be set to true unless the remaining byte
count is zero anyway.

Furthermore, on at least x86-64, if we do an early bailout, we don't
zero the entire buffer in the case of a hard-coded 10- or 16-byte buffer
(why only those sizes is anybody's guess.)  See lines 71-88 of uaccess_64.h.

I'd like to figure out what is the required and what is the desirable
behavior here, and then fix the code accordingly.

	-hpa