LinuxKPI: Better handling of zero-size and off-the-end reads

[jhb]

Summary

I noticed that the buf_size < 0 check can never be true (it's a
size_t) and decided to check for this condition by an alternate
expression, and I also noticed that a read_size of 0 would incorrectly
return -EFAULT. Instead, return success for both of these cases as
reading beyond the EOF of a normal file also returns EOF, not EINVAL.

Sponsored by: AFRL, DARPA

[jhb]

I probably could rearrange this to merge it before the change to switch to user pointers.

[jhb]

Though the EFAULT error is only after reverting the memcpy back to copy_from_user.