Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r238374:
Add a test program, written by Stephan Uphoff, which demonstrates the
deadlock due to i/o performed over the buffers backed by file mappings.

Add a test program, written by Stephan Uphoff, which demonstrates the
deadlock due to i/o performed over the buffers backed by file mappings.

MFC after: 2 weeks
Approved by: ups