simplify vss_read_chunk()

simplify vss_read_chunk()

All its parameters are known globally within vss.c, so omit them.
Also, return an int instead of the pointer to the inbuffer which is
also known to the caller.

This patch also introduces para_fseek() in fd.c which is used by
mp3.c and vss.c and changes the mp3 audio format handler so that it
uses the new function and always check the return value.