exec: introduce exec_binprm() for "depth == 0" code

exec: introduce exec_binprm() for "depth == 0" code

task_pid_nr_ns() and trace/ptrace code in the middle of the recursive
search_binary_handler() looks confusing and imho annoying.  We only need
this code if "depth == 0", lets add a simple helper which calls
search_binary_handler() and does trace_sched_process_exec() +
ptrace_event().

The patch also moves the setting of task->did_exec, we need to do this
only once.

Note: we can kill either task->did_exec or PF_FORKNOEXEC.

Signed-off-by: Oleg Nesterov <[email protected]>
Acked-by: Kees Cook <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Evgeniy Polyakov <[email protected]>
Cc: Zach Levis <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>