[8u] RFA: 8168628/8171452: (fc) SIGBUS when extending file size to map it

[David Buck]

Hi!

Please approve this backport of jdk8168628 to JDK 8. As jdk8168628 
introduced a regression (jdk8171452), I have also included the small fix 
for that in my backport as it makes sense to push them both at the same 
time.

bug report (original bug):
[ (fc) SIGBUS when extending file size to map it ]
https://bugs.openjdk.java.net/browse/JDK-8168628

bug report (minor regression fix added to above fix):
[ (ch) linux io_util_md: Operation not supported exception after 8168628 ]
https://bugs.openjdk.java.net/browse/JDK-8171452

code review of backport:
http://mail.openjdk.java.net/pipermail/nio-dev/2018-February/004673.html

I have tested the backport extensively, including building all platforms
supported by JPRT for JDK 8, several runs of the "core" jprt testset,
and manually confirming the fix with the test case included in the
changeset itself (both with and without RAF).

Cheers,
-Buck