pack-objects: merge read_lock and lock in packing_data struct

pack-objects: merge read_lock and lock in packing_data struct

Rename the packing_data lock to obd_lock and upgrade it to a recursive
mutex to make it suitable for current read_lock usages. Additionally
remove the superfluous #ifndef NO_PTHREADS guard around mutex
initialization in prepare_packing_data as the mutex functions
themselves are already protected.

Signed-off-by: Patrick Hogg <redacted>
Helped-by: Junio C Hamano <redacted>
Signed-off-by: Junio C Hamano <redacted>