[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/function/f_fs.c

@@ -949,7 +949,7 @@ static ssize_t ffs_epfile_io(struct file *file, struct ffs_io_data *io_data)
 			goto error_mutex;
 		}
 		if (!io_data->read &&
-		    copy_from_iter(data, data_len, &io_data->data) != data_len) {
+		    !copy_from_iter_full(data, data_len, &io_data->data)) {
 			ret = -EFAULT;
 			goto error_mutex;
 		}