client: Fix races in symlink attack detection for isrdir
client: Fix races in symlink attack detection for isrdir

- Do all of the isrdir symlink-attack checks on the same stat buffer
to prevent an attacker from changing the filesystem to evade the checks

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr