common/buffer: fix crc_map types

Signed-off-by: Sage Weil <sage@inktank.com>
common/buffer: invalidate crc on zero, copy_in

This does not capture users who

 - calc a crc
 - use c_str() to modify the buffer content
 - (re)calc a crc

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