layout_blas_general does not have an _extents member

Fix for issue kokkos#299 where we try to assign to _extents instead
of assigning to _base_layout.extents

Signed-off-by: Luc Berger-Vergiat <lberge@sandia.gov>

Author: lucbv

include/experimental/__p1673_bits/layout_blas_general.hpp

@@ -104,7 +104,7 @@ class __layout_blas_impl {
   MDSPAN_INLINE_FUNCTION MDSPAN_IMPL_CONSTEXPR_14
   __layout_blas_impl& operator=(__layout_blas_impl<OtherExtents, OtherLDA> const& other)
   {
-    this->_extents = other.extents();
+    this->_base_layout.extents = other.extents();
     this->__lda = other.__lda.value;
     return *this;
   }