[CORE] block: Keep track number of pending partd work

The patch proposes more generic and correct methode of partition
parsing by keeping track of pending partd work count.

The commit 61e13136f7c94a5259bd47a15983b795e3ecc31a "[CORE] block:
Don't add back block device for reparsing partitions" only solves
one part of the problem. We also need to keep in mind that partiton
parser module can be registered after a block device is created
hence we cannot simply forget about block devices for which parsing
fails. The new method introduced by this patch solves both these
problems cleanly.

Signed-off-by: Anup Patel <anup@brainfault.org>