cmake: find keyutils if rbd or libcephfs is enabled

cmake: find keyutils if rbd or libcephfs is enabled

* find_package(keyutils REQUIRED) if (WITH_LIBCEPHFS OR WITH_RBD)
  prior to this change, we detect keyutils if the building platform is not
  FreeBSD, we should instead check the WITH_* options, and let the
  maintainer to decided what is the best for his/her platform, and error
  out if the building host cannot fulfill the requirement to build the
  asserts.
* build krbd.cc if (WITH_RBD)

Signed-off-by: Kefu Chai <kchai@redhat.com>