ext2: convert byte order of constant instead of variable

Convert byte order of constant instead of variable which can be done at
compile time (vs run time).

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>