ext4: pack ioend structure better

On 64-bit architectures we have two 4-byte holes in struct ext4_io_end.
Order entries better to avoid this and thus make the structure occupy
64 instead of 72 bytes for 64-bit architectures.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>