iov_iter: fix bad parenthesis placement for iter_type check

iov_iter: fix bad parenthesis placement for iter_type check

Due to some unfortunate placement of the parenthesis for the iter_type
check in iov_iter_restore(), we can generate spurious triggers of the
type WARN_ON_ONCE() even if the iter is of the correct type.

While in there, correct the comment on what types can be used with the
save/restore helpers, and fix an extra word in the function description.

Fixes: 6696361cc3d8 ("new iov_iter flavour - ITER_UBUF")
Signed-off-by: Jens Axboe <axboe@kernel.dk>