knfsd: Replace lock_kernel with a mutex for nfsd thread startup/shutdown locking.

This removes the BKL from the RPC service creation codepath. The BKL
really isn't adequate for this job since some of this info needs
protection across sleeps.

Also, add some comments to try and clarify how the locking should work
and to make it clear that the BKL isn't necessary as long as there is
adequate locking between tasks when touching the svc_serv fields.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
diff --git a/include/linux/nfsd/nfsd.h b/include/linux/nfsd/nfsd.h
index 41d30c9..88d85b9 100644
--- a/include/linux/nfsd/nfsd.h
+++ b/include/linux/nfsd/nfsd.h
@@ -54,6 +54,7 @@
 extern struct svc_program	nfsd_program;
 extern struct svc_version	nfsd_version2, nfsd_version3,
 				nfsd_version4;
+extern struct mutex		nfsd_mutex;
 extern struct svc_serv		*nfsd_serv;
 
 extern struct seq_operations nfs_exports_op;