9p: Remove unneeded cast from memory allocation

Remove kmem_cache_alloc return value cast.

Coccinelle emits the following warning:

./fs/9p/vfs_inode.c:226:12-29: WARNING: casting value returned by memory allocation function to (struct v9fs_inode *) is useless.

Link: http://lkml.kernel.org/r/1596013140-49744-1-git-send-email-liheng40@huawei.com
Signed-off-by: Li Heng <liheng40@huawei.com>
[Dominique: commit message wording]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

fs/9p/vfs_inode.c

@@ -223,8 +223,7 @@ v9fs_blank_wstat(struct p9_wstat *wstat)
 struct inode *v9fs_alloc_inode(struct super_block *sb)
 {
 	struct v9fs_inode *v9inode;
-	v9inode = (struct v9fs_inode *)kmem_cache_alloc(v9fs_inode_cache,
-							GFP_KERNEL);
+	v9inode = kmem_cache_alloc(v9fs_inode_cache, GFP_KERNEL);
 	if (!v9inode)
 		return NULL;
 #ifdef CONFIG_9P_FSCACHE