We were returning 0 in places where the output was being truncated. I
couldn't find any consumers of KERN_PROC_CWD, but kinfo_getfile(3)
pretty clearly shouldn't be returning a truncated file array.
This change is incomplete, as most callers of export_vnode_to_sb() do
not check the return value and thus don't detect truncation. It's
sufficient to fix KERN_PROC_CWD though.