ext4: Fix include/trace/events/ext4.h to work with Systemtap
Using relative pathnames in #include statements interacts badly with
SystemTap, since the fs/ext4/*.h header files are not packaged up as
part of a distribution kernel's header files. Since systemtap doesn't
use TP_fast_assign(), we can use a blind structure definition and then
make sure the needed header files are defined before the ext4 source
files #include the trace/events/ext4.h header file.