mtx: get rid of file/line args from slow paths if they are unused

This denotes changes which went in by accident in r313877.

On most production kernels both said parameters are zeroed and have nothing
reading them in either mtx_lock_sleep or mtx_unlock_sleep. Thus this change
stops passing them by internal consumers which this is the case.

Kernel modules use _flags variants which are not affected kbi-wise.