iterate_and_advance(): get rid of magic in case when n is 0

iov_iter_advance() needs to do some non-trivial work when it's given
0 as argument (skip all empty iovecs, mostly).  We used to implement
it via iterate_and_advance(); we no longer do so and for all other
users of iterate_and_advance() zero length is a no-op.

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