vfs: 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.

Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

mm/filemap.c

@@ -2115,6 +2115,7 @@ void iov_iter_advance(struct iov_iter *i, size_t bytes)
 	} else {
 		const struct iovec *iov = i->iov;
 		size_t base = i->iov_offset;
+		unsigned long nr_segs = i->nr_segs;
 
 		/*
 		 * The !iov->iov_len check ensures we skip over unlikely
@@ -2130,11 +2131,13 @@ void iov_iter_advance(struct iov_iter *i, size_t bytes)
 			base += copy;
 			if (iov->iov_len == base) {
 				iov++;
+				nr_segs--;
 				base = 0;
 			}
 		}
 		i->iov = iov;
 		i->iov_offset = base;
+		i->nr_segs = nr_segs;
 	}
 }
 EXPORT_SYMBOL(iov_iter_advance);