lib: radix_tree.c make percpu variable static

radix_tree_preloads is unused outside of this file, make it static.

Noticed by sparse:
lib/radix-tree.c:84:1: warning: symbol 'per_cpu__radix_tree_preloads' was not declared. Should it be static?

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>