Fetching from user space should be done in a non-atomic context.  So
use a per-cpu buffer and copy its content to the ring buffer
atomically.  Note that we can migrate during accessing user memory
thus use a per-cpu mutex to protect concurrent accesses.

This is needed since we'll be able to fetch args from an user memory
which can be swapped out.  Before that uprobes could fetch args from
registers only which saved in a kernel space.

While at it, use __get_data_size() and store_trace_args() to reduce
code duplication.  And add struct uprobe_cpu_buffer and its helpers as
suggested by Oleg.
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: zhangwei(Jovi) <jovi.zhangwei@huawei.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>