Add implementation of createNonRecursive, needed by HBase

Signed-off-by: Mike Bryant <mike@mikebryant.me.uk>
cephfs: fixup returned exception types

Driven by HDFS unit test behavior.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
cephfs: clean-up opened file descriptor

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>