[mlir][linalg] Fix bufferize pattern to allow unknown operations in body of generic

The original version of the bufferization pattern for linalg.generic would
manually clone operations within the region to the bufferized clone of the
operation. This triggers legality requirements on those operations in the
conversion infra. Instead, this now uses the rewriter to inline the region
instead, avoiding those legality requirements.

Differential Revision: https://reviews.llvm.org/D109581