zfs/vfs lockups, via poudriere

[Sean Bruno]

bdrewery reported a vfs/zfs condition where operations will stall out
and block (rm, mv, file) during a poudriere build.  I've hit this now
and it seems to be alleviated by setting vfs.lookup_shared=0

I seem to be able to trivially reproduce this on my builders and want to
know if anyone is looking to diagnose this further.

original message:
https://lists.freebsd.org/pipermail/freebsd-fs/2014-September/020035.html

On my builders I see:

procstat -kka | grep zfs

    0 100666 kernel           zfs_vn_rele_task mi_switch+0xe1 sleepq_wait+0x3a _sleep+0x2ad taskqueue_thread_loop+0xf5 fork_exit+0x9a fork_trampoline+0xe 
    3 100151 zfskern          arc_reclaim_thre mi_switch+0xe1 sleepq_timedwait+0x3a _cv_timedwait_sbt+0x1ad arc_reclaim_thread+0x288 fork_exit+0x9a fork_trampoline+0xe 
    3 100152 zfskern          l2arc_feed_threa mi_switch+0xe1 sleepq_timedwait+0x3a _cv_timedwait_sbt+0x1ad l2arc_feed_thread+0x16f fork_exit+0x9a fork_trampoline+0xe 
    3 100657 zfskern          trim zroot       mi_switch+0xe1 sleepq_timedwait+0x3a _cv_timedwait_sbt+0x1ad trim_thread+0x9e fork_exit+0x9a fork_trampoline+0xe 
    3 100675 zfskern          txg_thread_enter mi_switch+0xe1 sleepq_wait+0x3a _cv_wait+0x190 txg_quiesce_thread+0x39b fork_exit+0x9a fork_trampoline+0xe 
    3 100676 zfskern          txg_thread_enter mi_switch+0xe1 sleepq_timedwait+0x3a _cv_timedwait_sbt+0x1ad txg_sync_thread+0x1dc fork_exit+0x9a fork_trampoline+0xe 
31071 100995 rm               -                mi_switch+0xe1 sleepq_wait+0x3a sleeplk+0x18d __lockmgr_args+0x9ab vop_stdlock+0x3c VOP_LOCK1_APV+0xab _vn_lock+0x43 zfs_lookup+0x45d zfs_freebsd_lookup+0x6d VOP_CACHEDLOOKUP_APV+0xa1 vfs_cache_lookup+0xd6 VOP_LOOKUP_APV+0xa1 lookup+0x5a1 namei+0x534 kern_rmdirat+0x8d amd64_syscall+0x3fb Xfast_syscall+0xfb 
31075 100693 mv               -                mi_switch+0xe1 sleepq_wait+0x3a sleeplk+0x18d __lockmgr_args+0xd5d vop_stdlock+0x3c VOP_LOCK1_APV+0xab _vn_lock+0x4
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