force_flush_memtable_and_lzero_now always compacts to physical L1

[mdcallag]

force_flush_memtable_and_lzero_now always compacts to the physical L1, which might not be in use when dynamic leveled is enabled. In that case it should compact to the logical L1, which might be the level named L2 or L3.

The buggy code, written by me, is here for 5.6.35 and 8.0.28.

Instead of "1" the argument to CompactFiles can be determined via a call to getProperty using the "rocksdb.base-level" property, or kBaseLevel. See here.