ida: do zeroing in ida_pre_get()

As far as I can tell, the only place the per-cpu ida_bitmap is populated
is in ida_pre_get.  The pre-allocated element is stolen in two places in
ida_get_new_above, in both cases immediately followed by a memset(0).

Since ida_get_new_above is called with locks held, do the zeroing in
ida_pre_get, or rather let kmalloc() do it.  Also, apparently gcc
generates ~44 bytes of code to do a memset(, 0, 128):

  $ scripts/bloat-o-meter vmlinux.{0,1}
  add/remove: 0/0 grow/shrink: 2/1 up/down: 5/-88 (-83)
  Function                                     old     new   delta
  ida_pre_get                                  115     119      +4
  vermagic                                      27      28      +1
  ida_get_new_above                            715     627     -88

Link: http://lkml.kernel.org/r/20180108225634.15340-1-linux@rasmusvillemoes.dkSigned-off-by: NRasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: NMatthew Wilcox <mawilcox@microsoft.com>
Cc: Eric Biggers <ebiggers@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/idr.c

@@ -431,7 +431,6 @@ int ida_get_new_above(struct ida *ida, int start, int *id)
 			bitmap = this_cpu_xchg(ida_bitmap, NULL);
 			if (!bitmap)
 				return -EAGAIN;
-			memset(bitmap, 0, sizeof(*bitmap));
 			bitmap->bitmap[0] = tmp >> RADIX_TREE_EXCEPTIONAL_SHIFT;
 			rcu_assign_pointer(*slot, bitmap);
 		}
@@ -464,7 +463,6 @@ int ida_get_new_above(struct ida *ida, int start, int *id)
 			bitmap = this_cpu_xchg(ida_bitmap, NULL);
 			if (!bitmap)
 				return -EAGAIN;
-			memset(bitmap, 0, sizeof(*bitmap));
 			__set_bit(bit, bitmap->bitmap);
 			radix_tree_iter_replace(root, &iter, slot, bitmap);
 		}

lib/radix-tree.c

@@ -2125,7 +2125,7 @@ int ida_pre_get(struct ida *ida, gfp_t gfp)
 		preempt_enable();
 
 	if (!this_cpu_read(ida_bitmap)) {
-		struct ida_bitmap *bitmap = kmalloc(sizeof(*bitmap), gfp);
+		struct ida_bitmap *bitmap = kzalloc(sizeof(*bitmap), gfp);
 		if (!bitmap)
 			return 0;
 		if (this_cpu_cmpxchg(ida_bitmap, NULL, bitmap))