get rid of legacy 'get_ds()' function

Every in-kernel use of this function defined it to KERNEL_DS (either as
an actual define, or as an inline function).  It's an entirely
historical artifact, and long long long ago used to actually read the
segment selector valueof '%ds' on x86.

Which in the kernel is always KERNEL_DS.

Inspired by a patch from Jann Horn that just did this for a very small
subset of users (the ones in fs/), along with Al who suggested a script.
I then just took it to the logical extreme and removed all the remaining
gunk.

Roughly scripted with

   git grep -l '(get_ds())' -- :^tools/ | xargs sed -i 's/(get_ds())/(KERNEL_DS)/'
   git grep -lw 'get_ds' -- :^tools/ | xargs sed -i '/^#define get_ds()/d'

plus manual fixups to remove a few unusual usage patterns, the couple of
inline function cases and to fix up a comment that had become stale.

The 'get_ds()' function remains in an x86 kvm selftest, since in user
space it actually does something relevant.

Inspired-by: Jann Horn <[email protected]>
Inspired-by: Al Viro <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

security/integrity/iint.c

@@ -200,7 +200,7 @@ int integrity_kernel_read(struct file *file, loff_t offset,
 		return -EBADF;
 
 	old_fs = get_fs();
-	set_fs(get_ds());
+	set_fs(KERNEL_DS);
 	ret = __vfs_read(file, buf, count, &offset);
 	set_fs(old_fs);