btrfs: rename extent map shrinker members from struct btrfs_fs_info

The names for the members of struct btrfs_fs_info related to the extent
map shrinker are a bit too long, so rename them to be shorter by replacing
the "extent_map_" prefix with the "em_" prefix.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>