vfs: Remove vfs_xattr_cmp

This function was only briefly used in security/integrity/evm, between
commits 66dbc325 and 15647eb3.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>