LU-14805 llite: No locked parallel DIO

LU-14805 llite: No locked parallel DIO

If we are doing locked DIO, the OSC & LDLM locks are
released at the end of cl_io_loop, ie, before we wait for
parallel DIO at the llite layer.

This is problematic because the locks are released before
i/o done using them is complete; this can lead to data
inconsistencies.  (And at least one LBUG, see LU-14805.)

The easiest solution for now is only do parallel DIO when
working lockless (which is the default; DIO only switches
to locked to manage conflicts with buffered i/o).

This problem & fix apply to AIO as well as parallel DIO.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: If98a0551d6dde54220b406b26e978e284a6b1ebf