ext4: Fix comment about the 64BIT feature

64BIT is part of the incompatible feature set, update the comment
accordingly.

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20230301133842.671821-1-tudor.ambarus@linaro.org


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