Avoid gcc -Werror=maybe-uninitialized 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.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1244