mdadm/util: unify fstat checking blkdev into function

mdadm/util: unify fstat checking blkdev into function

declare function fstat_is_blkdev() to integrate repeated fstat
checking block device operations, it returns true/1 when it is
a block device, and returns false/0 when it isn't.
The fd and devname are necessary parameters, *rdev is optional,
parse the pointer of dev_t *rdev, if valid, assigned the 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>