[REGRESSION] mm: process_vm_readv testcase no longer works after compat_prcoess_vm_readv removed

[Kyle Huey]

A test program from the rr[0] test suite, vm_readv_writev[1], no
longer works on 5.10-rc1 when compiled as a 32 bit binary and executed
on a 64 bit kernel. The first process_vm_readv call (on line 35) now
fails with EFAULT. I have bisected this to
c3973b401ef2b0b8005f8074a10e96e3ea093823.

It should be fairly straightforward to extract the test case from our
repository into a standalone program.

- Kyle

[0] https://rr-project.org/
[1] https://github.com/mozilla/rr/blob/master/src/test/vm_readv_writev.c