common/sctp_crc32: accept NULL buffer pointer

If we get NULL we behave as if it were a zero-filled buffer.

Signed-off-by: Sage Weil <sage@inktank.com>
common/crc32c_intel_baseline: accept null data buffer

Signed-off-by: Sage Weil <sage@inktank.com>
common/crc32c: document interface

Signed-off-by: Sage Weil <sage@inktank.com>
common/buffer: pass NULL buffer instead of a zero-filled one

This is about 2x faster on my box using the sctp code.

Signed-off-by: Sage Weil <sage@inktank.com>