su: Prevent stack overflow in check_perms

su: Prevent stack overflow in check_perms

This is no real world security fix.

The overflow could occur if too many layered subsystems are encountered
because the function check_perms calls itself recursively.

It would already take a misconfigured system for this to achieve it.

Use an iterative approach by calling the do_check_perms in a loop
instead of calling itself recursively.

As a side note: At least GCC 13 optimized this code and already uses
a jmp in its assembler code. I could only see the stack overflow by
activating address sanitizer which prevented the optimization.

Co-developed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Samanta Navarro <ferivoz@riseup.net>