mm: iov_iter: have iov_iter_advance() decrement nr_segs appropriately

mm: iov_iter: have iov_iter_advance() decrement nr_segs appropriately

Currently, when you call iov_iter_advance, then the pointer to the iovec
array can be incremented, but it does not decrement the nr_segs value in
the iov_iter struct.  The result is a iov_iter struct with a nr_segs value
that goes beyond the end of the array.

While I'm not aware of anything that's specifically broken by this, it
seems odd and a bit dangerous not to decrement that value.  If someone
were to trust the nr_segs value to be correct, then they could end up
walking off the end of the array.

Changing this might also provide some micro-optimization when dealing with
the last iovec in an array.  Many of the other routines that deal with
iov_iter have optimized codepaths when nr_segs == 1.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>