mmap: replace NOPAGE_SIGBUS with NULL

Doing so as it no longer is defined and it was equal to NULL anyway.

Signed-off-by: Salym Senyonga <salymsash@gmail.com>
mmap: Replace vm_ops.nopage() with vm_ops.fault()

Replace vm_operations_struct.nopage with vm_operations_struct.fault as
the nopage function was dropped and much of its functionality is
implemented by the fault method

Signed-off-by: Salym Senyonga <salymsash@gmail.com>