sna: Rename the two variants of pwrite so very relative dangers are obvious

sna: Rename the two variants of pwrite so very relative dangers are obvious

Since we extend the write in the cache-aligned routine, it runs the risk
of reading from beyond the end of the allocation. As such, callers
should be carefully vetted to make sure that their allocations are
already cache-aligned (typically page-aligned). To make it obvious that
this complexity exists, rename the routine.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>