fix braino in generic_file_read_iter()

Wrong sign of iov_iter_revert() argument.  Unfortunately, slipped through
the testing, since most of the time we don't do anything to the iterator
afterwards and potential oops on walking the iter->iov too far backwards
is too infrequent to be easily triggered.

Add a sanity check in iov_iter_revert() to catch bugs like this one;
fortunately, the same braino hadn't happened in other callers, but we'd
better have a warning if such thing crops up.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -790,6 +790,8 @@ void iov_iter_revert(struct iov_iter *i, size_t unroll)
 {
 	if (!unroll)
 		return;
+	if (WARN_ON(unroll > MAX_RW_COUNT))
+		return;
 	i->count += unroll;
 	if (unlikely(i->type & ITER_PIPE)) {
 		struct pipe_inode_info *pipe = i->pipe;

mm/filemap.c

@@ -2048,7 +2048,7 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 			iocb->ki_pos += retval;
 			count -= retval;
 		}
-		iov_iter_revert(iter, iov_iter_count(iter) - count);
+		iov_iter_revert(iter, count - iov_iter_count(iter));
 
 		/*
 		 * Btrfs can have a short DIO read if we encounter