lib/pthreadpool: fix a memory leak

lib/pthreadpool: fix a memory leak

When copying large files from the server to the client with aio enabled
we noticed that smbd kept growing RSS and VSZ.

valgrind was reporting:

==2503== 4,093,440 bytes in 6,560 blocks are possibly lost in loss record 460 of 460
==2503==    at 0x4C299CE: calloc (vg_replace_malloc.c:711)
==2503==    by 0x4011C24: _dl_allocate_tls (in /usr/lib64/ld-2.17.so)
==2503==    by 0x4E3C960: pthread_create@@GLIBC_2.2.5 (in /usr/lib64/libpthread-2.17.so)
==2503==    by 0x9B298AE: pthreadpool_add_job (in /usr/lib64/samba/libmessages-dgm-samba4.so)
==2503==    by 0x9B29FDC: pthreadpool_tevent_job_send (in /usr/lib64/samba/libmessages-dgm-samba4.so)
==2503==    by 0x56A78EF: ??? (in /usr/lib64/samba/libsmbd-base-samba4.so)
==2503==    by 0x55D86B7: smb_vfs_call_pread_send (in /usr/lib64/samba/libsmbd-base-samba4.so)
==2503==    by 0x55F7543: schedule_smb2_aio_read (in /usr/lib64/samba/libsmbd-base-samba4.so)
==2503==    by 0x5608F57: smbd_smb2_request_process_read (in /usr/lib64/samba/libsmbd-base-samba4.so)
==2503==    by 0x55FCB6C: smbd_smb2_request_dispatch (in /usr/lib64/samba/libsmbd-base-samba4.so)
==2503==    by 0x55FD7DC: ??? (in /usr/lib64/samba/libsmbd-base-samba4.so)
==2503==    by 0x641B977: ??? (in /usr/lib64/samba/libtevent.so.0.9.31)

The problem seems to be caused by worked threads that are not properly
started in detached state and thus their tls is not reclaimed upon
thread termination.

In pthreadpool.c we prepare a pthread attribute with
PTHREAD_CREATE_DETACHED, but we don't pass it to pthread_create().

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12624

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Fri Mar 10 22:06:02 CET 2017 on sn-devel-144