ceph: fix xattr rbtree search

Fix xattr name comparison in rbtree search for strings that share a prefix.
The *name argument is null terminated, but the xattr name is not, so we
need to use strncmp, but that means adjusting for the case where name is
a prefix of xattr->name.

The corresponding case in __set_xattr() already handles this properly
(although in that case *name is also not null terminated).

Reported-by: Sergiy Kibrik <[e-mail]>
Signed-off-by: Sage Weil <[e-mail]>

fs/ceph/xattr.c

···
 	struct rb_node **p;
 	struct rb_node *parent = NULL;
 	struct ceph_inode_xattr *xattr = NULL;
+	int name_len = strlen(name);
 	int c;
 
 	p = &ci->i_xattrs.index.rb_node;
···
 		parent = *p;
 		xattr = rb_entry(parent, struct ceph_inode_xattr, node);
 		c = strncmp(name, xattr->name, xattr->name_len);
+		if (c == 0 && name_len > xattr->name_len)
+			c = 1;
 		if (c < 0)
 			p = &(*p)->rb_left;
 		else if (c > 0)