printk: remove unneeded dead-store assignment

make clang-analyzer on x86_64 defconfig caught my attention with:

  kernel/printk/printk_ringbuffer.c:885:3: warning:
  Value stored to 'desc' is never read [clang-analyzer-deadcode.DeadStores]

		desc = to_desc(desc_ring, head_id);
		^

Commit b6cf8b3f33 ("printk: add lockless ringbuffer") introduced
desc_reserve() with this unneeded dead-store assignment.

As discussed with John Ogness privately, this is probably just some minor
left-over from previous iterations of the ringbuffer implementation. So,
simply remove this unneeded dead assignment to make clang-analyzer happy.

As compilers will detect this unneeded assignment and optimize this anyway,
the resulting object code is identical before and after this change.

No functional change. No change to object code.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20201106034005.18822-1-lukas.bulwahn@gmail.com

kernel/printk/printk_ringbuffer.c

@@ -882,8 +882,6 @@ static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
 	head_id = atomic_long_read(&desc_ring->head_id); /* LMM(desc_reserve:A) */
 
 	do {
-		desc = to_desc(desc_ring, head_id);
-
 		id = DESC_ID(head_id + 1);
 		id_prev_wrap = DESC_ID_PREV_WRAP(desc_ring, id);