i40e/base: fix arq_event_info struct

Overloading the 'msg_size' field in the 'arq_event_info' struct
is a bad idea. It leads to bugs when the structure is used in a
loop, since the input value (buffer size) is overwritten by the
output value (actual message length). The fix introduces one
more field of 'buf_len' for the buffer size, and renames the
field of 'msg_size' to 'msg_len' for the real message size.

Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Reviewed-by: Chen Jing <jing.d.chen@intel.com>
Tested-by: HuilongX Xu <huilongx.xu@intel.com>