ext4: Move the ext4_i.h header file into ext4.h

There is no longer a reason for a separate ext4_i.h header file, so
move it into ext4.h just to make life easier for developers to find
the relevant data structures and typedefs.  Should also speed up
compiles slightly, too.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>