stream_smb: increase to 128k read_chuuk from default 8k

Previous to this commit, read_chunk was not set in stream_smb. The
cache was therefore filled in small 8K chunks. This resulted in poor
performance when compared to, for example, smbnetfs on the same
network.

The value of 128k is chosen both because it is emperically
the "levelling off point" for throughput into mpv's cache, and because
it is the value chosen by smbnetfs when serving smb shares to
mpv.

Note that this change has no effect unless --cache is explicitly
specified as smb:// streams do not activate cache by default. This is
because the default cache size of 320K is so small it actually makes
smb:// perfomance worse. For best results use at least --cache=1024.