lightnvm: pblk: make symbol write_buffer_size static

Fixes the following sparse warning:

drivers/lightnvm/pblk-init.c:23:14: warning:
 symbol 'write_buffer_size' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/lightnvm/pblk-init.c

@@ -20,7 +20,7 @@
 
 #include "pblk.h"
 
-unsigned int write_buffer_size;
+static unsigned int write_buffer_size;
 
 module_param(write_buffer_size, uint, 0644);
 MODULE_PARM_DESC(write_buffer_size, "number of entries in a write buffer");