From: Brian Behlendorf <behlendorf1@llnl.gov>
Subject: [PATCH] fix stack vn_open

We should not put a 4k maxpathlen buffer on the stack, instead
locate it to the heap.  Even in user space we run ztest with 8K
stacks to verify correctness

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