[CORE] block: Don't add back block device for reparsing partitions

Currently, if partition parsing fails for a block device then we
add block device back to partd work queue. This is incorrect because
it breaks in a situation where we have more than one block devices.

Let us take an example of two block devices "a" and "b" added to
partd work queue where partition parsing succeeds for "a" but fails
for "b" hence "b" is added back to partd work queue. Now in future,
if some new block device "c" is added then current partd will only
parse "b" again and miss the parsing of "c" block device.

The patch simplifies and fixes block device partition parsing.

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