(fc) Modify sun.nio.ch.FileChannelImpl.map0() to accept a FileDescriptor parameter

Currently the Java_sun_nio_ch_FileChannelImpl_map0() function obtains the FileDescriptor from the 'fd' Object field of the sun.nio.ch.FileChannelImpl instance. It would be better simply to pass the FileDescriptor object as a parameter to map0(). This will also dispense with the native global variable 'chan_fd'.