Bug: MappedMemorySegmentImpl uses internal FileChannelImpl

[Ty Young]

Tried using Google's in-memory filesystem:


https://github.com/google/jimfs


for storing mmap files but ran into an issue because 
MappedMemorySegmentImpl uses internal FileChannelImpl method:


https://github.com/openjdk/panama-foreign/blob/acc11115db42cc0e17efdf361d58c65f4e922f39/src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/MappedMemorySegmentImpl.java#L105


which causes a class casting exception when using Google's in-memory 
Java filesystem and probably other implementations as well.