docs: Make syscalls' helpers naming consistent

The documentation explains the need to create internal syscalls' helpers,
and that they should be called `kern_xyzzy()`. However, the comment at
include/linux/syscalls.h says that they should be named as
`ksys_xyzzy()`, and so are all the helpers declared bellow it. Change the
documentation to reflect this.

Fixes: 819671ff849b ("syscalls: define and explain goal to not call syscalls in the kernel")
Signed-off-by: André Almeida <[e-mail]>
Reviewed-by: Dominik Brodowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/process/adding-syscalls.rst

···
 useful to be used within the kernel, needs to be shared between an old and a
 new syscall, or needs to be shared between a syscall and its compatibility
 variant, it should be implemented by means of a "helper" function (such as
-``kern_xyzzy()``).  This kernel function may then be called within the
+``ksys_xyzzy()``).  This kernel function may then be called within the
 syscall stub (``sys_xyzzy()``), the compatibility syscall stub
 (``compat_sys_xyzzy()``), and/or other kernel code.