f2fs: xattr.h: Make stub helpers inline

Fix gcc warnings:

In file included from fs/f2fs/dir.c:15:0:
fs/f2fs/xattr.h:157:13: warning: 'f2fs_destroy_xattr_caches' defined but not used [-Wunused-function]
 static void f2fs_destroy_xattr_caches(struct f2fs_sb_info *sbi) { }
             ^~~~~~~~~~~~~~~~~~~~~~~~~
fs/f2fs/xattr.h:156:12: warning: 'f2fs_init_xattr_caches' defined but not used [-Wunused-function]
 static int f2fs_init_xattr_caches(struct f2fs_sb_info *sbi) { return 0; }

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: a999150f4fe3 ("f2fs: use kmem_cache pool during inline xattr lookups")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>