nfsd: fix oops when reading pool_stats before server is started

Sourbh reported an oops that is triggerable by trying to read the
pool_stats procfile before nfsd had been started. Move the check for a
NULL serv in svc_pool_stats_start above the mutex acquisition, and fix
the stop routine not to unlock the mutex if there is no serv yet.

Fixes: 7b207ccd

 ("svc: don't hold reference for poolstats, only mutex.")
Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>