On AVR32, all parameters beyond the 5th are passed on the stack. System
calls don't use the stack -- they borrow a callee-saved register
instead. This means that syscalls that take 6 parameters must be called
through a stub that pushes the last parameter on the stack.
This relates to syscall fallocate, fanotify_mark, process_vm_readv, and
process_vm_writev.