Avoid gcc warnings.

Explicitly set acl details to zero to silence gcc (zfs_acl_node_read
can't be sure zfs_acl_znode_info will set acl_count and aclsize).
Normally suppressing these warnings by setting this to zero at
declaration time is a bad idea but in this instance it's hard to avoid
and should be fairly safe.

(It's reasonable to reject this until it's clear why I need this and other don't, the commit message could probably be better as well)