ext4: add comments to definition of ext4_io_end_t

This should make it more clear what this structure is used
for, and how some of the (mutually exclusive) fields are
used to keep page cache references.

Signed-off-by: Curt Wohlgemuth <curtw@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>