Fix races during _mesa_HashWalk().

Fix races during _mesa_HashWalk().

There is currently no protection against walking a hash (using
_mesa_HashWalk()) and modifying it at the same time, for instance by inserting
or deleting elements. This leads to segfaults in multithreaded code if e.g.
someone calls glTexImage2D (which may have to walk the list of FBOs) while
another thread is calling glDeleteFramebuffers on another thread with the two
contexts sharing lists.

The reason for this is that _mesa_HashWalk() doesn't actually take the mutex
that normally protects the hash; it takes an entirely different mutex.
Thus, walks are only protected against other walks, and there is also no
outer lock taking this. There is an old comment saying that this is to fix
problems with deadlock if the callback needs to take a mutex; we solve this
by changing the mutex to be recursive.

A demonstration Helgrind hit from a real application:

==13412== Possible data race during write of size 8 at 0x3498C6A8 by thread #1
==13412== Locks held: 2, at addresses 0x1AF09530 0x2B3DF400
==13412==    at 0x1F040C99: _mesa_hash_table_remove (hash_table.c:395)
==13412==    by 0x1EE98174: _mesa_HashRemove_unlocked (hash.c:350)
==13412==    by 0x1EE98174: _mesa_HashRemove (hash.c:365)
==13412==    by 0x1EE2372D: _mesa_DeleteFramebuffers (fbobject.c:2669)
==13412==    by 0x6105AA4: movit::ResourcePool::cleanup_unlinked_fbos(void*) (resource_pool.cpp:473)
==13412==    by 0x610615B: movit::ResourcePool::release_fbo(unsigned int) (resource_pool.cpp:442)
[...]
==13412== This conflicts with a previous read of size 8 by thread #20
==13412== Locks held: 2, at addresses 0x1AF09558 0x1AF73318
==13412==    at 0x1F040CD9: _mesa_hash_table_next_entry (hash_table.c:415)
==13412==    by 0x1EE982A8: _mesa_HashWalk (hash.c:426)
==13412==    by 0x1EED6DFD: _mesa_update_fbo_texture.part.33 (teximage.c:2683)
==13412==    by 0x1EED9410: _mesa_update_fbo_texture (teximage.c:3043)
==13412==    by 0x1EED9410: teximage (teximage.c:3073)
==13412==    by 0x1EEDA28F: _mesa_TexImage2D (teximage.c:3105)
==13412==    by 0x166A68: operator() (mixer.cpp:454)

There are many more interactions than just these two possible.

Cc: 11.2 12.0 13.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Steinar H. Gunderson <steinar+mesa@gunderson.no>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>