accel/tcg: Unlock mmap_lock after longjmp

accel/tcg: Unlock mmap_lock after longjmp

The mmap_lock is held around tb_gen_code.  While the comment
is correct that the lock is dropped when tb_gen_code runs out
of memory, the lock is *not* dropped when an exception is
raised reading code for translation.

Acked-by: Alistair Francis <alistair.francis@wdc.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>