zfs: fix mismatch between format specifier and type

vdev_dbgmsg_print_tree printed vdev_id of uint64_t type with %u format
specifier. That caused subsequent parameters to be incorrectly read
from the stack and lead to a crash when a wrong value was interpreted as
a string pointer.

This should be upstreamed.

Reported by: pho
MFC after: 3 days