ipvs: Fix possible deadlock in sync code

Commit 998e7a76 ("ipvs: Use kthread_run()
instead of doing a double-fork via kernel_thread()") introduced a possible
deadlock in the sync code. We need to use the _bh versions for the lock, as the
lock is also accessed from a bottom half.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Acked-by: Simon Horman <horms@verge.net.au>