md: introduce link/unlink_rdev() helpers

There are places where sysfs links to rdev are handled
in a same way. Add the helper functions to consolidate
them.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

diff --cc drivers/md/md.c
Simple merge

diff --cc drivers/md/md.h
Simple merge

diff --cc drivers/md/raid1.c
Simple merge

diff --cc drivers/md/raid5.c
Simple merge