btrfs: switch check_direct_IO() to iov_iter

... and don't open-code iov_iter_alignment() there
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/btrfs/inode.c

@@ -7391,39 +7391,30 @@ static void btrfs_submit_direct(int rw, struct bio *dio_bio,
 }
 
 static ssize_t check_direct_IO(struct btrfs_root *root, int rw, struct kiocb *iocb,
-			const struct iovec *iov, loff_t offset,
-			unsigned long nr_segs)
+			const struct iov_iter *iter, loff_t offset)
 {
 	int seg;
 	int i;
-	size_t size;
-	unsigned long addr;
 	unsigned blocksize_mask = root->sectorsize - 1;
 	ssize_t retval = -EINVAL;
-	loff_t end = offset;
 
 	if (offset & blocksize_mask)
 		goto out;
 
-	/* Check the memory alignment.  Blocks cannot straddle pages */
-	for (seg = 0; seg < nr_segs; seg++) {
-		addr = (unsigned long)iov[seg].iov_base;
-		size = iov[seg].iov_len;
-		end += size;
-		if ((addr & blocksize_mask) || (size & blocksize_mask))
-			goto out;
-
-		/* If this is a write we don't need to check anymore */
-		if (rw & WRITE)
-			continue;
+	if (iov_iter_alignment(iter) & blocksize_mask)
+		goto out;
 
-		/*
-		 * Check to make sure we don't have duplicate iov_base's in this
-		 * iovec, if so return EINVAL, otherwise we'll get csum errors
-		 * when reading back.
-		 */
-		for (i = seg + 1; i < nr_segs; i++) {
-			if (iov[seg].iov_base == iov[i].iov_base)
+	/* If this is a write we don't need to check anymore */
+	if (rw & WRITE)
+		return 0;
+	/*
+	 * Check to make sure we don't have duplicate iov_base's in this
+	 * iovec, if so return EINVAL, otherwise we'll get csum errors
+	 * when reading back.
+	 */
+	for (seg = 0; seg < iter->nr_segs; seg++) {
+		for (i = seg + 1; i < iter->nr_segs; i++) {
+			if (iter->iov[seg].iov_base == iter->iov[i].iov_base)
 				goto out;
 		}
 	}
@@ -7443,8 +7434,7 @@ static ssize_t btrfs_direct_IO(int rw, struct kiocb *iocb,
 	bool relock = false;
 	ssize_t ret;
 
-	if (check_direct_IO(BTRFS_I(inode)->root, rw, iocb, iter->iov,
-			    offset, iter->nr_segs))
+	if (check_direct_IO(BTRFS_I(inode)->root, rw, iocb, iter, offset))
 		return 0;
 
 	atomic_inc(&inode->i_dio_count);