fix brown paperbag bug in inlined copy_..._iter()

"copied nothing" == "return 0", not "return full size".

Fixes: aa28de27

 "iov_iter/hardening: move object size checks to inlined part"
Spotted-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>