ACPICA: Fix for possible fault in acpi_ex_release_mutex

Fixed a problem with the AML Mutex handling function
acpi_ex_release_mutex where the function could fault under the very
rare condition when the interpreter has blocked, the interpreter
lock is released, the interpreter is then reentered via the
same thread, and attempts to acquire a mutex that was previously
acquired. FreeBSD report 140979.

http://www.freebsd.org/cgi/query-pr.cgi?pr=140979



Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>