cmake: detect linux/blk/zoned support

cmake: detect linux/blk/zoned support

* add find_package() support for detecting the existence of
  linux/blkzoned.h before using it.
* link against Linux::ZNS for adding the compilation definition of
  HAVE_ZNS instead of including it in the global config.h
* move the CMake option closer to where it is used for
  better readability. as this option takes effect only if
  crimson is compiled.
* make WITH_ZNS an option which depends on the value of
  CMAKE_SYSTEM_NAME. it is hidden and off if CMAKE_SYSTEM_NAME
  is not "Linux".

Signed-off-by: Kefu Chai <tchaikov@gmail.com>