libext2fs: fix Direct I/O support in the Unix I/O manager

libext2fs: fix Direct I/O support in the Unix I/O manager

Commit d557b9659ba9 ("libext2fs: fix potential races in unix_io")
contained some additional changes that were supposed to clarify the
management of the length calculations when reading and writing
non-page aligned data using Direct I/O; unfortunately, those changes
were incorrect, and actually broke the Direct I/O support.

Fix these problems by reverting those portions of that commit.

[ commit description rewritten by tytso ]

Originally from https://github.com/tytso/e2fsprogs/pull/68

Fixes: d557b9659ba9 ("libext2fs: fix potential races in unix_io")
Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>