btrfs: remove redundant readonly root check in btrfs_setxattr_trans

btrfs_setxattr_trans() is called by 5 functions as below and all of them
do updates. None of them would be roun on a read-only root.
So its ok to remove the readonly root check here as it's a high-level
conditon.

1.
  __btrfs_set_acl()
    btrfs_init_acl()
      btrfs_init_inode_security()

2.
  __btrfs_set_acl()
    btrfs_set_acl()

3.
  btrfs_set_prop()
    btrfs_set_prop_trans()
      /                       \
      btrfs_ioctl_setflags()   btrfs_xattr_handler_set_prop()

4.
  btrfs_xattr_handler_set()

5.
  btrfs_initxattrs()
    btrfs_xattr_security_init()
      btrfs_init_inode_security()
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/xattr.c

@@ -226,9 +226,6 @@ int btrfs_setxattr_trans(struct btrfs_trans_handle *trans,
 	struct btrfs_root *root = BTRFS_I(inode)->root;
 	int ret;
 
-	if (btrfs_root_readonly(root))
-		return -EROFS;
-
 	if (trans)
 		return btrfs_setxattr(trans, inode, name, value, size, flags);