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 3661d286

.

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>