staging: lustre: Fix sparse warning for one-bit signed bitfield

Fix the following sparse warning:
drivers/staging/lustre/lustre/llite/llite_internal.h:461:49: error:dubious one-bit signed bitfield
Signed-off-by: Dulshani Gunawardhana <dulshani.gunawardhana89@gmail.com>
Reviewed-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/llite/llite_internal.h

@@ -458,7 +458,7 @@ struct ll_sb_info {
 	struct lu_fid	     ll_root_fid; /* root object fid */
 
 	int		       ll_flags;
-	int			  ll_umounting:1;
+	unsigned int			  ll_umounting:1;
 	struct list_head		ll_conn_chain; /* per-conn chain of SBs */
 	struct lustre_client_ocd  ll_lco;