fs/proc01.c: Add known issue.

Since 786d7e1612f0b0adb6046f19b906609e4fe8b1ba upstream kernel commit
the proc operations on regular files are done via proc_reg_*() proxies
so the default errno for unimplemented read on proc file changed to EIO
returned from the proxy rather than EINVAL that was previously returned
from vfs_read().

The only affected known issue I've found is the '/proc/xen/privcmd'
file which makes the testcase fail on certain xen installations.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>