fbarray: reduce duplication in chunk finding
fbarray: reduce duplication in chunk finding

Mostly code move, aside from more quick checks done to avoid
doing computations in obviously hopeless cases.

Signed-off-by: Anatoly Burakov <814e00d8f6e4e81b7ccc7e3e5a0bfec0078f0203@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk