iov_iter_gap_alignment(): get rid of iterate_all_kinds()

iov_iter_gap_alignment(): get rid of iterate_all_kinds()

For one thing, it's only used for iovec (and makes sense only for those).
For another, here we don't care about iov_offset, since the beginning of
the first segment and the end of the last one are ignored.  So it makes
a lot more sense to just walk through the iovec array...

We need to deal with the case of truncated iov_iter, but unlike the
situation with iov_iter_alignment() we don't care where the last
segment ends - just which segment is the last one.

[fixed a braino spotted by Qian Cai <quic_qiancai@quicinc.com>]

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