fix: Shift exponent too large

Undefined operation is reported by running ztest (or zloop) compiled with GCC
UndefinedBehaviorSanitizer. Error only happens on top level of dnode indirection
with large enough offset values. Logically, left shift operation would work,
but bit shift semantics in C, and limitation of uint64_t, do not produce desired
result.

Issue #5059, #4883

Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>