syscall.c: always update tcp->currpers in update_personality

syscall.c: always update tcp->currpers in update_personality

Sometimes (for example, switching from a process with one personality
to a process that previously had different personality but returning
from execve to that same personality into) it is possible that
current_personality is not changed, but tcp->currpers is different.
So, let's not return from update_personality and always update
tcp->currpers if it differs from the target personality.

* syscall.c (update_personality): Do not exit early if personality ==
current_personality.