fix a race between del_gendisk and BLKRRPART

Hi all,

this is based of a patch from Gulam and suggestions from Ming and fixes a
race between del_gendisk and BLKRRPART while also removing a global lock.


Diffstat:
 block/genhd.c         |   19 ++-----------------
 fs/block_dev.c        |   12 ++++--------
 include/linux/genhd.h |    2 --
 3 files changed, 6 insertions(+), 27 deletions(-)