btrfs: fix ASSERT em->len condition in btrfs_get_extent

The em->len value is supposed to be verified in the assertion condition
though we expect it to be same as the sectorsize.

Fixes: a196a894 ("btrfs: do not reset extent map members for inline extents read")
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Tanmay Bhushan <007047221b@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>