There was a race conditon in the reference counting of file descriptors
during a close() operation. After the call to the close handler, the
rtems_libio_free() function cleared the flags to zero. However, at this
point in time there may still exist some holders of the file descriptor.
With RTEMS_DEBUG enabled this could lead to failed assertions in
rtems_libio_iop_drop().
Change the code to use only atomic read-modify-write operations on the
rtems_libio_iop::flags.
c2287ba2cf