sstables: leveled_manifest: prevent benign precision loss warning
sstables: leveled_manifest: prevent benign precision loss warning

Casting from the maximum int64_t to double loses precision, because
int64_t has 64 bits of precision while double has only 53. Clang
warns about it. Since it's not a real problem here, add an explicit
cast to silence the warning.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla