Bump do_write buffer size

In the case of do_write (write to generic file) there
is really no relation to the PROP_VALUE_MAX limitation
of system properties. The current value, 92, is not so
much for writing to files so bump it a bit.

Ideally there should not be any low hard limit in this
case. Either expand_props should allocate the output
buffer or take the output fd and to streaming write.
Such a change is rather invasive though and at this
point in time it seems reasonable that one dont want
more than 256 character strings in the init files.

Change-Id: I846a282ae4e747e6171eef38b302b46287951451