vol: Fix ntops to provide expected semantics

vol: Fix ntops to provide expected semantics

nt_open() can be asked to open a file with a path containing
directory components that do not yet exist.  Modify nt_open()
to automatically create directories if necessary.

nt_read() and nt_pread() should not treat partial reads due
to reaching EOF as an error.  Instead, return the bytes read.

Change-Id: If088393a842d6f6d7ad901345e94e92d23f2321c
Reviewed-on: http://gerrit.openafs.org/3666
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>