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 <[e-mail]>
Fixes: a999150f4fe3 ("f2fs: use kmem_cache pool during inline xattr lookups")
Signed-off-by: YueHaibing <[e-mail]>
Reviewed-by: Chao Yu <[e-mail]>
Signed-off-by: Jaegeuk Kim <[e-mail]>

fs/f2fs/xattr.h

···
 {
 	return -EOPNOTSUPP;
 }
-static int f2fs_init_xattr_caches(struct f2fs_sb_info *sbi) { return 0; }
-static void f2fs_destroy_xattr_caches(struct f2fs_sb_info *sbi) { }
+static inline int f2fs_init_xattr_caches(struct f2fs_sb_info *sbi) { return 0; }
+static inline void f2fs_destroy_xattr_caches(struct f2fs_sb_info *sbi) { }
 #endif
 
 #ifdef CONFIG_F2FS_FS_SECURITY