LU-15317 llite: Add D_IOTRACE

LU-15317 llite: Add D_IOTRACE

In looking in to performance problems, it's very important
to be able to trace the I/O patterns from userspace in to
Lustre, and also understand the key basics of how Lustre
handles that I/O (readahead, RPC generation).

This is best done with a dedicated debug flag - No
userspace tool can provide all this information, and
existing debug flags collect a huge number of unrelated
pieces of, well, debug information.

The goal is for customers to be able to quickly gather log
files of a reasonable size which contain the necessary
information and which can easily be interpreted by
engineering.  This is not possible if the information is
spread out across a number of heavyweight debug flags.

This is a first pass at adding the flag and the debug
required to track basic data I/O.  One significant
omission in the first patch is RPC generation - I have not
decided how best to do that yet.  That will be added in a
future patch.

test-parameters: trivial

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I0ed003ec1488e1c267b194c871f64b34f6dc6025