ext4: fix compile with CONFIG_EXT4_FS_XATTR disabled

Commit 5dabfc78dced ("ext4: rename {exit,init}_ext4_*() to
ext4_{exit,init}_*()") causes

  fs/ext4/super.c:4776: error: implicit declaration of function ‘ext4_init_xattr’

when CONFIG_EXT4_FS_XATTR is disabled.

It renamed init_ext4_xattr to ext4_init_xattr but forgot to update the
dummy definition in fs/ext4/xattr.h.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ext4/xattr.h

@@ -122,7 +122,7 @@ ext4_xattr_put_super(struct super_block *sb)
 }
 
 static __init inline int
-init_ext4_xattr(void)
+ext4_init_xattr(void)
 {
 	return 0;
 }