ext2: No longer export ext2_fs.h to user space

ext2: No longer export ext2_fs.h to user space

Since the on-disk format has been stable for quite some time, users
should either use the headers provided by libext2fs or keep a private
copy of this header. For the full discussion, see this thread:

	https://lkml.org/lkml/2012/3/21/516

While at it, this commit removes all __KERNEL__ guards, which are now
unnecessary.

Signed-off-by: Thierry Reding <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Ted Ts'o <[email protected]>
Cc: Artem Bityutskiy <[email protected]>
Cc: Andreas Dilger <[email protected]>
Cc: [email protected]