net: Add alignment support for net_buf data length alloc

The alignment value tells the amount of alignment of buffer length
when allocating net_buf data for sending. By default there is no
special alignment. This is needed for example with Nordic Wi-Fi
chip that uses SPI driver that expects 4 byte alignment for the
length of the data that is being sent.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>