Avoid unnecessary lseek() calls by cleanups in md.c. mdfd_lstbcnt was

Avoid unnecessary lseek() calls by cleanups in md.c.  mdfd_lstbcnt was
not being consulted anywhere, so remove it and remove the _mdnblocks()
calls that were used to set it.  Change smgrextend interface to pass in
the target block number (ie, current file length) --- the caller always
knows this already, having already done smgrnblocks(), so it's silly to
do it over again inside mdextend.  Net result: extension of a file now
takes one lseek(SEEK_END) and a write(), not three lseeks and a write.