f2fs: avoid checkpatch error

ERROR:INITIALISED_STATIC: do not initialise statics to NULL
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/compress.c

@@ -506,7 +506,7 @@ bool f2fs_is_compress_backend_ready(struct inode *inode)
 	return f2fs_cops[F2FS_I(inode)->i_compress_algorithm];
 }
 
-static mempool_t *compress_page_pool = NULL;
+static mempool_t *compress_page_pool;
 static int num_compress_pages = 512;
 module_param(num_compress_pages, uint, 0444);
 MODULE_PARM_DESC(num_compress_pages,