btrfs: remove superfluous metadata check at btrfs_lookup_extent_info()

If we didn't found an extent item with the initial btrfs_search_slot()
call, it's pointless to test if the "metadata" variable is "true", because
right after we check if the key type is BTRFS_METADATA_ITEM_KEY and that
is the case only when "metadata" is set to "true". So remove the redundant
check.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>