metag/usercopy: Switch to RAW_COPY_USER

Switch to using raw user copy instead of providing metag specific
[__]copy_{to,from}_user[_inatomic](). This simplifies the metag
uaccess.h and allows us to take advantage of extra checking in the
generic versions.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-metag@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>