generic compat_sys_ustat

Due to a different size of ino_t ustat needs a compat handler, but
currently only x86 and mips provide one.  Add a generic compat_sys_ustat
and switch all architectures over to it.  Instead of doing various
user copy hacks compat_sys_ustat just reimplements sys_ustat as
it's trivial.  This was suggested by Arnd Bergmann.

Found by Eric Sandeen when running xfstests/017 on ppc64, which causes
stack smashing warnings on RHEL/Fedora due to the too large amount of
data writen by the syscall.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/arch/mips/kernel/scall64-o32.S b/arch/mips/kernel/scall64-o32.S
index b0fef4f..60997f1 100644
--- a/arch/mips/kernel/scall64-o32.S
+++ b/arch/mips/kernel/scall64-o32.S
@@ -265,7 +265,7 @@
 	PTR	sys_olduname
 	PTR	sys_umask			/* 4060 */
 	PTR	sys_chroot
-	PTR	sys_32_ustat
+	PTR	compat_sys_ustat
 	PTR	sys_dup2
 	PTR	sys_getppid
 	PTR	sys_getpgrp			/* 4065 */