Style. Add comment about lock mode.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

sys/fs/tmpfs/tmpfs_subr.c

@@ -548,6 +548,7 @@ loop1:
 		goto unlock;
 	MPASS(vp != NULL);
 
+	/* lkflag is ignored, the lock is exclusive */
 	(void) vn_lock(vp, lkflag | LK_RETRY);
 
 	vp->v_data = node;

sys/fs/tmpfs/tmpfs_vnops.c

@@ -171,7 +171,7 @@ tmpfs_lookup(struct vop_cachedlookup_args *v)
 
 				/* Allocate a new vnode on the matching entry. */
 				error = tmpfs_alloc_vp(dvp->v_mount, tnode,
-						cnp->cn_lkflags, vpp);
+				    cnp->cn_lkflags, vpp);
 				if (error != 0)
 					goto out;