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>

arch/x86/ia32/ia32entry.S

@@ -557,7 +557,7 @@ ia32_sys_call_table:
 	.quad sys32_olduname
 	.quad sys_umask		/* 60 */
 	.quad sys_chroot
-	.quad sys32_ustat
+	.quad compat_sys_ustat
 	.quad sys_dup2
 	.quad sys_getppid
 	.quad sys_getpgrp		/* 65 */