[PATCH] block_dev.c mutex_lock_nested() fix

In the case below we are locking the whole disk not a partition.  This
change simply brings the code in line with the piece above where when we
are the 'first' opener, and we are a partition.

Signed-off-by: Jason Baron <jbaron@redhat.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>