ext4: Add a stub for mpage_da_data in the trace header

The tracepoint ext4_da_write_pages has a struct mpage_da_data*
parameter, but that struct is only defined in fs/ext4/ext4.h.  This
patch adds a forward declaration for that struct, so this tracepoint
header can still be used by tools like SystemTap.

This is a continuation of the fix in commit 3661d28.

http://sourceware.org/bugzilla/show_bug.cgi?id=10703

Signed-off-by: Josh Stone <jistone@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

include/trace/events/ext4.h

@@ -11,6 +11,7 @@ struct ext4_allocation_context;
 struct ext4_allocation_request;
 struct ext4_prealloc_space;
 struct ext4_inode_info;
+struct mpage_da_data;
 
 #define EXT4_I(inode) (container_of(inode, struct ext4_inode_info, vfs_inode))