nfs: use safe, interruptible sleeps when waiting to retry LOCK

We actually want to use TASK_INTERRUPTIBLE sleeps when we're in the
process of polling for a NFSv4 lock. If there is a signal pending when
the task wakes up, then we'll be returning an error anyway. So, we might
as well wake up immediately for non-fatal signals as well. That allows
us to return to userland more quickly in that case, but won't change the
error that userland sees.

Also, there is no need to use the *_unsafe sleep variants here, as no
vfs-layer locks should be held at this point.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>