MINOR: fd: add fd_get_running() to atomically return the running mask

MINOR: fd: add fd_get_running() to atomically return the running mask

The running mask is only valid if the tgid is the expected one. This
function takes a reference on the tgid before reading the running mask,
so that both are checked at once. It returns either the mask or zero if
the tgid differs, thus providing a simple way for a caller to check if
it still holds the FD.