shortcut to the process of the passed lwp paniced the kernel since lwp
could/can be passwd as NULL in VOP_WRITE().
This was happening when ktracing to NFS. The function ktrwrite() set the
uio_lwp to NULL and then calls VOP_WRITE() with this argument. nfs_write()
then accessed lwp *l->l_proc wich paniced.
Thanks to David Laight for his help on tracking it down.
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