common/null_blk: allow _configure_null_blk with built-in null_blk

common/null_blk: allow _configure_null_blk with built-in null_blk

Test that do use _configure_null_blk already configure do not actually
require a built-in null_blk driver.  Relax the check in _have_null_blk
to just require a driver instead of a module, and instead set a skip
reason instead of failing in _init_null_blk when null_blk is built-in
or otherwise not available.

Also try to load the null_blk module in _configure_null_blk as
_init_null_blk is optional now and we thus can't rely on the module
to be loaded.

Signed-off-by: Christoph Hellwig <hch@lst.de>