blk/kernel: use uint64_t for bdev_flock_retry

blk/kernel: use uint64_t for bdev_flock_retry

also drop bdev_flock_retry and bdev_flock_retry_interval from
legacy_config_opts.h, as `KernelDevice::_lock()` is not in the critical
path, there is no need to access these settings via member variables --
get_val<> would just suffice.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 8fb80db9ff32a85fcdf91e38075c42165805e37f)

Conflicts:
	src/blk/kernel/KernelDevice.cc
- file does not exist in nautilus; changes applied without conflicts to
  src/os/bluestore/KernelDevice.cc