mdadm/util: unify stat checking blkdev into function

mdadm/util: unify stat checking blkdev into function

declare function stat_is_blkdev() to integrate repeated stat
checking blkdev operations, it returns 'true/1' when it is a
block device, and returns 'false/0' when it isn't.
The devname is necessary parameter, *rdev is optional, parse
the pointer of dev_t *rdev, if valid, assigned device number
to dev_t *rdev, if NULL, ignores.

Signed-off-by: Zhilong Liu <zlliu@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>