It is possible that an osc_extent contains more than 256 chunks, and
the IO engine won't add this extent in one RPC
(try_to_add_extent_for_io) so that osc_check_rpcs() run into a loop
upon this extent and never break.
This patch changes osc_max_write_chunks() to make sure the value
can cover all possible osc_extent, so that all osc_extent will be
added into one RPC.
This patch also add another field erd_max_extents in extent_rpc_data
to make sure not to add too many fragments in a single RPC.