btrfs: remove dead code from btrfs_get_extent

Due to new_inline logic, the create == 0 is always true at this
point in the code, so the create != 0 branch can be removed.

Signed-off-by: Edmund Nadolski <enadolski@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>