mm/vmalloc: use rb_tree instead of list for vread() lookups

vread() has been linearly searching vmap_area_list for looking up vmalloc
areas to read from.  These same areas are also tracked by a rb_tree
(vmap_area_root) which offers logarithmic lookup.

This patch modifies vread() to use the rb_tree structure instead of the
list and the speedup for heavy /proc/kcore readers can be pretty
significant.  Below are the wall clock measurements of a Python
application that leverages the drgn debugging library to read and
interpret data read from /proc/kcore.

Before the patch:
-----
  $ time sudo sdb -e 'dbuf | head 3000 | wc'
  (unsigned long)3000

  real	0m22.446s
  user	0m2.321s
  sys	0m20.690s
-----

With the patch:
-----
  $ time sudo sdb -e 'dbuf | head 3000 | wc'
  (unsigned long)3000

  real	0m2.104s
  user	0m2.043s
  sys	0m0.921s
-----

Link: https://lkml.kernel.org/r/20210209190253.108763-1-serapheim@delphix.comSigned-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmalloc.c

@@ -2894,7 +2894,10 @@ long vread(char *buf, char *addr, unsigned long count)
 		count = -(unsigned long) addr;
 
 	spin_lock(&vmap_area_lock);
-	list_for_each_entry(va, &vmap_area_list, list) {
+	va = __find_vmap_area((unsigned long)addr);
+	if (!va)
+		goto finished;
+	list_for_each_entry_from(va, &vmap_area_list, list) {
 		if (!count)
 			break;