Revert "nfsd: fix oops when reading pool_stats before server is started"

This reverts commit 8e948c36.

The reverted commit moves a test on a field protected by a mutex outside
of the protection of that mutex, and so is obviously racey.

Depending on how the race goes, si->serv might be NULL when dereferenced
in svc_pool_stats_start(), or svc_pool_stats_stop() might unlock a mutex
that hadn't been locked.

This bug that the commit tried to fix has been addressed by initialising
->mutex earlier.

Fixes: 8e948c36

 ("nfsd: fix oops when reading pool_stats before server is started")
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>