FileChannelImpl: Ignore ftruncate failures on non-regular files

Truncate only works on regular files.  On other files, such as
UNIX character devices, truncate returns EINVAL, which
causes an IOException to be thrown.

With this change, we now have support for creating a MappedByteBuffer
on UNIX character devices such as /dev/zero, if the underlying
device node supports mmap().

Change-Id: I094371d821b187abe3da32edc411ff76a81b047e