cred_guard_mutex: do not return -EINTR to user-space

do_execve() and ptrace_attach() return -EINTR if
mutex_lock_interruptible(->cred_guard_mutex) fails.

This is not right, change the code to return ERESTARTNOINTR.

Perhaps we should also change proc_pid_attr_write().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>