rlimits: switch more rlimit syscalls to do_prlimit

After we added more generic do_prlimit, switch sys_getrlimit to that.
Also switch compat handling, so we can get rid of ugly __user casts
and avoid setting process' address limit to kernel data and back.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --cc kernel/compat.c
Simple merge

diff --cc kernel/sys.c
Simple merge