Ignore chunk size of SPA_DATA_DmaBuf

dmabufs have no well defined size, so what most producers provide are
some nonsensical values. Ideally, it should be zero.

This change makes kpipewire ignore the chunk size if the specified buffer
has dmabuf type. memfd and memptr code paths still check the chunk size
out of an abundance of caution.

dmabuf buffers that contain no data are expected to have CORRUPTED flag.

(cherry picked from commit 18551571)