Define ext2_listxattr to NULL when CONFIG_EROFS_FS_XATTR
is not enabled, then we can remove many ugly ifdef macros
in the code.

Link: https://lore.kernel.org/r/20200522044035.24190-2-cgxu519@mykernel.netSigned-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Jan Kara <jack@suse.cz>