md: Return error if request_module fails and returns positive value

request_module() can return 256 (process exited) in some cases,
which is not as specified in the documentation before the
request_module() definition. Convert the error to -ENOENT.

The positive error number results in bitmap_create() returning
a value that is meant to be an error but doesn't look like one,
so it is dereferenced as a point and causes a crash.

(not needed for stable as this is "experimental" code)
Fixes: edb39c9d ("Introduce md_cluster_operations to handle cluster functions")
Signed-off-By: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: NeilBrown <neilb@suse.com>