Check every entry in xattr block for validity in ext2_xattr_set() to
detect on disk corruption early. Also since e_value_block field in xattr
entry is never != 0 in a valid filesystem, just remove checks for it
once we have established entries are valid.
Reviewed-by: Chengguang Xu <cgxu519@zoho.com.cn>
Signed-off-by: Jan Kara <jack@suse.cz>