LU-13088 ldlm: Fix sleeping function called in atomic

LU-13088 ldlm: Fix sleeping function called in atomic

target_recovery_overseer() can sleep while holding a spinlock, which
triggers a BUG warning.

It is easily fixed by dropping the spinlock before waiting.  In the
case where the task waits, no useful information that could be
protected by the spinlock is held, so nothing can be lost by dropping
it.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I8bb3d02523b5dcfadac19f01ccb736d7b7f28239
Reviewed-on: https://review.whamcloud.com/37063
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>