net: buf: remove use of special putter and getter functions

Convert users of net_buf_put() and net_buf_get() functions to use
non-wrapped putters and getters k_fifo_put() and k_fifo_get().

Special handling of net_bufs in k_fifos is no longer needed after commit
3d306c18, since these actions are now
atomic regardless of any net_buf fragments.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>