MINOR: stats-file: use explicit unsigned integer bitshift for user slots

MINOR: stats-file: use explicit unsigned integer bitshift for user slots

As reported in GH #3104, there remained a place where (1 << shift was
used to set or remove bits from uint64_t users bitfield. It is incorrect
and could lead to bugs for values > 32 bits.

Instead, let's use 1ULL to ensure the operation remains 64bits consistent.

No backport needed.