btrfs: pass a valid extent map cache pointer to __get_extent_map()

We can pass a valid em cache pointer down to __get_extent_map() and
drop the validity check. This avoids the special case, the call stacks
are simple:

btrfs_read_folio
  btrfs_do_readpage
    __get_extent_map

extent_readahead
  contiguous_readpages
    btrfs_do_readpage
      __get_extent_map

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>