MIPS: checksum: Split the 'copy_user' symbol

The 'copy_user' symbol can be used to copy from or to
userland so we will use two different symbols for these
operations. This makes no difference in the existing code,
but when the core is operating in EVA mode, different instructions
need to be used to read and write to userland address space.
The old function has also been renamed to 'copy_kernel' to denote
that it is suitable for copy data to and from kernel space.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>