[RFC v1 1/2] mm: Reorganize SLAB freelist randomization

This commit reorganizes the previous SLAB freelist randomization to prepare for the SLUB implementation. It moves functions that will be shared to slab_common. It also move the definition of freelist_idx_t in the slab_def header so a similar type can be used for all common functions.

[RFC v1 1/2] mm: Reorganize SLAB freelist randomization

This commit reorganizes the previous SLAB freelist randomization to prepare for the SLUB implementation. It moves functions that will be shared to slab_common. It also move the definition of freelist_idx_t in the slab_def header so a similar type can be used for all common functions.