Fix inode->i_blksize (for fstat st_blksize and hopefully improved cp

Fix inode->i_blksize (for fstat st_blksize and hopefully improved cp
performance) so that it is the maximum vector write size (num_stripes *
max_iov_size) as told by obd_get_info("brw_size").

Also change the obd_get_info() API to fix some minor abuses (passing part
of the key in via val, returning data via the return code).  Also remove
any pretense of obd_get_info() allocating memory for the caller, following
normal portals caller-allocates behaviour.  Make inputs and returns be
fixed size, so they can be network-encoded across different arches.