->splice_write() via ->write_iter()

iter_file_splice_write() - a ->splice_write() instance that gathers the
pipe buffers, builds a bio_vec-based iov_iter covering those and feeds
it to ->write_iter().  A bunch of simple cases coverted to that...

[AV: fixed the braino spotted by Cyrill]

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

fs/exofs/file.c

@@ -77,7 +77,7 @@ const struct file_operations exofs_file_operations = {
 	.fsync		= exofs_file_fsync,
 	.flush		= exofs_flush,
 	.splice_read	= generic_file_splice_read,
-	.splice_write	= generic_file_splice_write,
+	.splice_write	= iter_file_splice_write,
 };
 
 const struct inode_operations exofs_file_inode_operations = {