xfs: untangle the direct I/O and DAX path, fix DAX locking

[Christoph Hellwig <hch@xxxxxx>]

The last patch is what started the series:  XFS currently uses the
direct I/O locking strategy for DAX because DAX was overloaded onto
the direct I/O path.  For XFS this means that we only take a shared
inode lock instead of the normal exclusive one for writes IFF they
are properly aligned.  While this is fine for O_DIRECT which requires
explicit opt-in from the application it's not fine for DAX where we'll
suddenly lose expected and required synchronization of the file system
happens to use DAX undeneath.

Patches 1-7 just untangle the code so that we can deal with DAX on
it's own easily.