This fix only affects UBI debugging.

If the the background thread is disabled for debugging purposes,
start it anyway, because otherwise we see tonns of kernel debugging
complaints like this:

INFO: task ubi_bgt0d:26857 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
ubi_bgt0d     D dd37bf94     0 26857      2
       dd37bfcc 00000086 f8e17cea dd37bf94 00000046 00000000 00000000 f5c62430
       f5c62430 f5c62590 c2a09c80 f6cbd498 dd8e9cbc 00000296 dd37bfb0 00000296
       dd8e9cb8 dd8e9cbc dd37bfcc c0119774 00000000 00000000 c0132e89 f6961560
Call Trace:
 [<f8e17cea>] ? ubi_thread+0x0/0x127 [ubi]
 [<c0119774>] ? complete+0x43/0x4b
 [<c0132e89>] ? kthread+0x0/0x5b
 [<f8e17cea>] ? ubi_thread+0x0/0x127 [ubi]
 [<c0132eae>] kthread+0x25/0x5b
 [<c0132e89>] ? kthread+0x0/0x5b
 [<c0104953>] kernel_thread_helper+0x7/0x14
 =======================

So start it, and go sleep inside it, instead of creating it and never
start.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>