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>