[ZFS] panic vm_fault : fault on no fault entry

Hello,

[8.0 BETA-4/i386]

I've got this panic when copying /usr on a new pool.
It's 100% reproductible here with 
tar cf - -C /usr . | tar xpvf - -C /pool

The panic occurs always with free() in the ZFS code.

Unfortunaly I'm not able to save the dump but it's like :
free()
zfs_acl_node_free()
zfs_acl_release_nodes()
zfs_acl_free()
zfs_zaccesss()
zfs_freebsd_create()
VOP_CREATE_APV()
vn_open_read()
vn_open()
kern_openat()
kern_open()
open()
syscall (open())

I saw this in the write() syscall too.

/boot/loader.conf
# ZFS
vm.kmem_size="512M"
vm.kmem_size_max="512M"
vfs.zfs.arc_max="40M"
vfs.zfs.vdev.cache.size="5M"

and options KVA_PAGES=512 in my kernel

Thanks in advance, regards.