This patch adds back O_DIRECT support with various caveats
attached:
1. Journaled data can be read via O_DIRECT since its now the
same on disk format as normal data files.
2. Journaled data writes with O_DIRECT will be failed sliently
back to normal writes (should we really do this I wonder or
should we return an error instead?)
3. Stuffed files will be failed back to normal buffered I/O
4. All the usual corner cases (write beyond current end of file,
write to an unallocated block) will also revert to normal buffered I/O.
The I/O path is slightly odd as reads arrive at the page cache layer
with the lock for the file already held, but writes arrive unlocked.
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
d1665e4142