In the quest to remove all stack VLA usage from the kernel[1], this moves
the buffer off the stack (since it could be as much as 1024 bytes), and
uses a new area in the cursor data structure. Additionally adds missed
documentation and removes redundant assignments.
[1]https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@xxxxxxxxxxxxxx
Signed-off-by: Kees Cook<keescook@xxxxxxxxxxxx>