[iov_iter] new primitives - copy_from_iter_full() and friends

copy_from_iter_full(), copy_from_iter_full_nocache() and
csum_and_copy_from_iter_full() - counterparts of copy_from_iter()
et.al., advancing iterator only in case of successful full copy
and returning whether it had been successful or not.

Convert some obvious users.  *NOTE* - do not blindly assume that
something is a good candidate for those unless you are sure that
not advancing iov_iter in failure case is the right thing in
this case.  Anything that does short read/short write kind of
stuff (or is in a loop, etc.) is unlikely to be a good one.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/usb/gadget/legacy/inode.c

@@ -667,7 +667,7 @@ ep_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		return -ENOMEM;
 	}
 
-	if (unlikely(copy_from_iter(buf, len, from) != len)) {
+	if (unlikely(!copy_from_iter_full(buf, len, from))) {
 		value = -EFAULT;
 		goto out;
 	}