extattr_list_vp: Only take shared vnode lock

List is a 'read'-type operation that does not modify shared state; it's safe
for multiple thread to proceed concurrently.  This is reflected in the vnode
operation LISTEXTATTR locking protocol specification, which only requires a
shared lock.

(Similar to previous r248933.)

Reported by:	Case van Rij <case.vanrij AT isilon.com>
Reviewed by:	kib, mjg
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D19082

sys/kern/vfs_extattr.c

@@ -633,7 +633,7 @@ extattr_list_vp(struct vnode *vp, int attrnamespace, void *data,
 	if (nbytes > IOSIZE_MAX)
 		return (EINVAL);
 
-	vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
+	vn_lock(vp, LK_SHARED | LK_RETRY);
 
 	auiop = NULL;
 	sizep = NULL;