Fix lock ordering in `fat_mutex_lock' to match that of `do_thread_exit'.

Fix lock ordering in `fat_mutex_lock' to match that of `do_thread_exit'.

Original Helgrind report:

==14160== Thread #57: lock order "0x47F6B90 before 0x7C25A28" violated
==14160==    at 0x4C27730: pthread_mutex_lock (in /.../valgrind-3.6.0/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==14160==    by 0x4EFF87E: do_thread_exit (threads.c:664)
==14160==    by 0x4E87B89: c_body (continuations.c:512)
==14160==    by 0x4F16C21: vm_regular_engine (vm-i-system.c:960)
==14160==    by 0x4E90F92: scm_call_4 (eval.c:506)
==14160==    by 0x4E88372: scm_i_with_continuation_barrier (continuations.c:450)
==14160==    by 0x4E88424: scm_c_with_continuation_barrier (continuations.c:546)
==14160==    by 0x51CA3AF: GC_call_with_gc_active (pthread_support.c:1128)
==14160==    by 0x4EFF2E0: with_guile_and_parent (threads.c:206)
==14160==    by 0x51C46B4: GC_call_with_stack_base (misc.c:1505)
==14160==    by 0x4EFF447: scm_with_guile (threads.c:917)
==14160==    by 0x51C46B4: GC_call_with_stack_base (misc.c:1505)
==14160==   Required order was established by acquisition of lock at 0x47F6B90
==14160==    at 0x4C27730: pthread_mutex_lock (in /.../valgrind-3.6.0/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==14160==    by 0x4F00262: fat_mutex_lock (threads.c:1362)
==14160==    by 0x4F004DA: scm_lock_mutex_timed (threads.c:1462)
==14160==    by 0x4F16C09: vm_regular_engine (vm-i-system.c:898)
==14160==    by 0x4E90F4D: scm_call_3 (eval.c:499)
==14160==    by 0x4EFFA4C: really_launch (threads.c:975)
==14160==    by 0x4E87B89: c_body (continuations.c:512)
==14160==    by 0x4F16C21: vm_regular_engine (vm-i-system.c:960)
==14160==    by 0x4E90F92: scm_call_4 (eval.c:506)
==14160==    by 0x4E88372: scm_i_with_continuation_barrier (continuations.c:450)
==14160==    by 0x4E88424: scm_c_with_continuation_barrier (continuations.c:546)
==14160==    by 0x4EFF289: with_guile_and_parent (threads.c:874)
==14160==   followed by a later acquisition of lock at 0x7C25A28
==14160==    at 0x4C27730: pthread_mutex_lock (in /.../valgrind-3.6.0/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==14160==    by 0x4F00352: fat_mutex_lock (threads.c:1374)
==14160==    by 0x4F004DA: scm_lock_mutex_timed (threads.c:1462)
==14160==    by 0x4F16C09: vm_regular_engine (vm-i-system.c:898)
==14160==    by 0x4E90F4D: scm_call_3 (eval.c:499)
==14160==    by 0x4EFFA4C: really_launch (threads.c:975)
==14160==    by 0x4E87B89: c_body (continuations.c:512)
==14160==    by 0x4F16C21: vm_regular_engine (vm-i-system.c:960)
==14160==    by 0x4E90F92: scm_call_4 (eval.c:506)
==14160==    by 0x4E88372: scm_i_with_continuation_barrier (continuations.c:450)
==14160==    by 0x4E88424: scm_c_with_continuation_barrier (continuations.c:546)
==14160==    by 0x4EFF289: with_guile_and_parent (threads.c:874)

* libguile/threads.c (fat_mutex_lock): In the `m->level == 0' case,
  release M's lock before taking T's `admin_mutex'.