uprobes: Change write_opcode() to use copy_*page()

Change write_opcode() to use copy_highpage() + copy_to_page()
and simplify the code.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Anton Arapov <anton@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>