Fix double shared memory allocation.

Fix double shared memory allocation.

SLRU buffer lwlocks are allocated twice by oversight in commit
fe702a7b3f9f2bc5bf6d173166d7d55226af82c8 where that locks were moved to
separate tranche. The bug doesn't have user-visible effects except small
overspending of shared memory.

Backpatch to 9.6 where it was introduced.

Alexander Korotkov with small editorization by me.