ca-cas-repack: Remove pointless call to lseek

sendfile takes an offset parameter, so we don't need to seek to the beginning
of the file before calling it.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
repack: Use link() to atomically add packs

Creating a hardlink using the link() system call never removes any existing
files.  This approach replaces access() + rename(), which suffers from a race
condition.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>