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This adds the basic support for Direct I/O to unix_io.c, and adds a
new flag EXT_FLAG_DIRECT_IO which can be passed to ext2fs_open() or
ext2fs_open2() to request Direct I/O support.

Note that device mapper devices in Linux don't support Direct I/O, and
in some circumstances using Direct I/O can actually make performance
*worse*!
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>