reftable/block: adjust type of the restart length

reftable/block: adjust type of the restart length

The restart length is tracked as a positive integer even though it
cannot ever be negative. Furthermore, it is effectively capped via the
MAX_RESTARTS variable.

Adjust the type of the variable to be `uint32_t`. While this type is
excessive given that MAX_RESTARTS fits into an `uint16_t`, other places
already use 32 bit integers for restarts, so this type is being more
consistent.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>