[mlir][bufferization] Restrict function boundary buffer. to func.call.

The current bufferization on function boundaries works on `func.func`
and any call op implementing `CallOpInterface`. Then, an error is thrown
if there is a `CallOpInterface` op that is not `func.call`. This is
unnecessary and breaks the pass whenever such an op occurs (such as
`llvm.call`). This PR simply restricts the handling of call ops to
`func.call`.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D143724