[mlir] rename the "packing" flag of linalg.pad_tensor to "nofold"

The discussion in https://reviews.llvm.org/D110425 demonstrated that "packing"
may be a confusing term to define the behavior of this op in presence of the
attribute. Instead, indicate the intended effect of preventing the folder from
being applied.

Reviewed By: nicolasvasilache, silvas

Differential Revision: https://reviews.llvm.org/D111046