Linux 4.1 compat: loop device on ZFS

Linux 4.1 compat: loop device on ZFS

Starting from Linux 4.1 allows iov_iter with bio_vec to be passed into
iter_read/iter_write. Notably, the loop device will pass bio_vec to backend
filesystem. However, current ZFS code assumes iovec without any check, so it
will always crash when using loop device.

With the restructured uio_t, we can safely pass bio_vec in uio_t with UIO_BVEC
set. The uio* functions are modified to handle bio_vec case separately.

The const uio_iov causes some warning in xuio related stuff, so explicit
convert them to non const.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3511
Closes #3640