Add CHECK_FOR_INTERRUPTS in Evict{Rel,All}UnpinnedBuffers

The pg_buffercache_evict_{relation,all} operations can become extremely
slow when dealing with large buffer pools containing many dirty buffers.
This commit adds CHECK_FOR_INTERRUPTS calls within the underlying
Evict{Rel,All}UnpinnedBuffers functions to ensure these operations
cancellable.

And this should be backpatched through v18 where
pg_buffercache_evict_{relation,all} operations are introduced.