ext4: keep uniform naming convention for io & io_end variables

Let's keep uniform naming convention for ext4_submit_io (io)
& ext4_end_io_t (io_end) structures, to avoid any confusion.
No functionality change in this patch.

Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/20191016073711.4141-2-riteshh@linux.ibm.com


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