Fix efile_drv crash when using async thread pool

When using the async thread pool and compressed files, when
an efile driver port instance is shutdown, the efile_drv
stop callback closes the file descriptor (a gzFile instance
actually) - this is dangerous if at the same time there's
an async thread performing an operation against the file,
for example calling invoke_read(), which can result in a
segmentation fault, or calling invoke_close() which double
closes the gzFile and this in turn causes 2 consecutive calls
to driver_free() against same gzFile instance (resulting in
later unexpected crashes in erl_bestfit_alloc.c for example).

The following test program makes the emulator crash when using
the async thread pool:

-module(t2).

-export([t/1]).

t(N) ->
    file:delete("foo.bar"),
    {ok, Fd} = file:open("foo.bar", [raw, write, binary, compressed]),
    ok = file:write(Fd, <<"qwerty">>),
    ok = file:close(Fd),
    Pid = spawn_link(fun() ->
       process_flag(trap_exit, true),
       loop(N)
    end),
    Ref = erlang:monitor(process, Pid),
    receive
    {'DOWN', Ref, _, _, _} ->
         ok
    end.

loop(0) ->
    ok;
loop(N) ->
    Server = self(),
    Pid = spawn(fun() ->
        {ok, Fd} = file:open("foo.bar", [read, raw, binary, compressed]),
        Server ! continue,
        _ = file:read(Fd, 5),
        file:close(Fd)
    end),
    receive continue -> ok end,
    exit(Pid, shutdown),
    loop(N - 1).

Running this test when using the async thread pool:

  shell> erl +A 4
  Erlang R15B03 (erts-5.9.3.1) [source] [64-bit] [smp:4:4] [async-threads:4] [hipe] [kernel-poll:false]

  Eshell V5.9.3.1  (abort with ^G)
  1> c(t2).
  {ok,t2}
  2> t2:t(10000).
  Segmentation fault (core dumped)

When not using the async thread pool, there are no issues:

  shell> erl
  Erlang R15B03 (erts-5.9.3.1) [source] [64-bit] [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]

  Eshell V5.9.3.1  (abort with ^G)
  1> c(t2).
  {ok,t2}
  2> t2:t(10000).
  ok
  3>

An example stack trace when the crash happens because there's
an ongoing read operation is:

  Thread 1 (Thread 0x7f021cf2c700 (LWP 10687)):
  #0  updatewindow (strm=0x2691bf8, out=5) at zlib/inflate.c:338
  #1  0x00000000005a2ba0 in inflate (strm=0x2691bf8, flush=0) at zlib/inflate.c:1141
  #2  0x000000000055c46a in erts_gzread (file=0x2691bf8, buf=0x7f0215b29e80, len=5) at drivers/common/gzio.c:523
  #3  0x00000000005849ef in invoke_read (data=0x26b2228) at drivers/common/efile_drv.c:1114
  #4  0x000000000050adcb in async_main (arg=0x7f021bf5cf40) at beam/erl_async.c:488
  #5  0x00000000005c21a0 in thr_wrapper (vtwd=0x7fff69c6ff10) at pthread/ethread.c:106
  #6  0x00007f021c573e9a in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
  #7  0x00007f021c097cbd in clone () from /lib/x86_64-linux-gnu/libc.so.6
  #8  0x0000000000000000 in ?? ()

And when there's an ongoing close operation when the driver
is stopped:

  Thread 1 (Thread 0x7fe5f5654700 (LWP 747)):
  #0  0x0000000000459b64 in bf_unlink_free_block (block=0x10b2a70, allctr=<optimized out>, flags=<optimized out>) at beam/erl_bestfit_alloc.c:792
  #1  bf_unlink_free_block (flags=0, block=0x10b2a70, allctr=0x873380) at beam/erl_bestfit_alloc.c:822
  #2  bf_get_free_block (allctr=0x873380, size=<optimized out>, cand_blk=<optimized out>, cand_size=<optimized out>, flags=0) at beam/erl_bestfit_alloc.c:869
  #3  0x000000000044f0dd in mbc_alloc_block (alcu_flgsp=<synthetic pointer>, blk_szp=<synthetic pointer>, size=200, allctr=0x873380) at beam/erl_alloc_util.c:1198
  #4  mbc_alloc (allctr=0x873380, size=200) at beam/erl_alloc_util.c:1345
  #5  0x000000000045449b in do_erts_alcu_alloc (size=200, extra=0x873380, type=165) at beam/erl_alloc_util.c:3442
  #6  erts_alcu_alloc_thr_pref (type=165, extra=<optimized out>, size=192) at beam/erl_alloc_util.c:3520
  #7  0x000000000055c0bf in gz_open (mode=0x5d98b2 "rb", path=0x1103418 "foo.bar") at drivers/common/gzio.c:164
  #8  erts_gzopen (path=0x1103418 "foo.bar", mode=0x5d98b2 "rb") at drivers/common/gzio.c:307
  #9  0x0000000000584e47 in invoke_open (data=0x1103330) at drivers/common/efile_drv.c:1857
  #10 0x000000000050adcb in async_main (arg=0x7fe5f698af80) at beam/erl_async.c:488