Cleanup mmap consuming

Only one mmap call per buffer, once the first mmap is done, we use the
offset to know where to start consuming the data.

When we receive the FD and its output mode is mmap, we mmap the buffer,
when it is closing, we munmap it.

Removed the manual padding handling because the kernel already exposes a
padded subbuffer.

Also updated the check of the return code of
kconsumerd_consumerd_recv_fd to be consistent with the rest of the code,
we are in error when it's < 0 not <= 0.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Julien Desfossez <julien.desfossez@polymtl.ca>