LU-12462 osc: Do not assert for first extent

LU-12462 osc: Do not assert for first extent

In the discard case, the OSC fsync/writeback code asserts
that each OSC extent is fully covered by the fsync request.

This is not valid for the DOM case, because OSC extent
alignment requirements can create OSC extents which start
before the OST region of the layout (ie, they cross in to
the DOM region).  This is OK because the layout prevents
them from ever being used for i/o, but this same behavior
means that the OSC fsync start/end is aligned with the
layout, and so does not necessarily cover that first
extent.

The simplest solution is just to not assert on the first
extent.  (There is no way at the OSC layer to recognize the
DOM case.)

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: If66f8d81fb9dd4546a5647a10f6ca551e2cf98e3
Reviewed-on: https://review.whamcloud.com/35525
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Andriy Skulysh <c17819@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>