bulk-checkin: fix sign compare warnings
In file bulk-checkin.c, three warnings are emitted by "-Wsign-compare", two of which are caused by trivial loop iterator type mismatches. The third one is also an uncomplicated case for which a simple cast is a sufficient remedy.
Fix issues accordingly, and enable sign compare warnings for the file.
Signed-off-by: Tuomas Ahola taahol@utu.fi