The meta lock now covers both meta data and data, so this just removes the
now-redundant data lock.

Combining locks saves us a round of lock mastery per inode and one less lock
to ping between nodes during read/write.

We don't lose much - since meta locks were always held before a data lock
(and at the same level) ordered writeout mode (the default) ensured that
flushing for the meta data lock also pushed out data anyways.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>