dm-lc.c: return -ENOMEM if alloc_workqueue fails

Audit the return value of alloc_workqueue and return -ENOMEM if it fails

Signed-off-by: Kumar Amit Mehta <gmate.amit@gmail.com>
Revert "persistent flag is added to segment_header. needless flushing of backing store is eliminated."

This reverts commit 5a7a340054b08c2645b8f843811fd59ea4a01d62.
minor mail fix