The search_xattrs ioctl is only going to find entries for xattrs with
the .srch. tag which create srch entries as they're created and
destroyed. Export the xattr tag parsing so that the ioctl can return
-EINVAL for xattrs which don't have the scoutfs prefix and the .srch.
tag.
Signed-off-by: Zach Brown <zab@versity.com>