net: Fix ip_buf_len after removing extra header

After removing extra header from IP packet, only uip_len is
updated not ip_buf_len. uip_len parameter is set to zero
at some point after handover to net_core. From net_core
only ip_buf_len holds the actual length. So update ip_buf_len
after removing extra header.

Change-Id: I2cc03ed1125df5216a0fa6e94eebfeee0a343481
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>