f2fs: xattr simplifications

Now that the xattr handler is passed to the xattr handler operations, we
have access to the attribute name prefix, so simplify
f2fs_xattr_generic_list.

Also, f2fs_xattr_advise_list is only ever called for
f2fs_xattr_advise_handler; there is no need to double check for that.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Changman Lee <cm224.lee@samsung.com>
Cc: Chao Yu <chao2.yu@samsung.com>
Cc: linux-f2fs-devel@lists.sourceforge.net
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>