LU-1757 osc: clarify short_io_bytes is maximum value

LU-1757 osc: clarify short_io_bytes is maximum value

Clarify in the code that the "osc.*.short_io_bytes" parameter is
the maximum IO size to pack into request/reply not the minimum.

Allow short_io to be disabled completely if it is set to zero.

It would be nice to also change the name of the /proc functions
in a similar manner but that also changes the /proc tunable name
(via LUSTRE_RW_ATTR() macro) and has compatibility implications
for sites that may have changed this value.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I3a91420d7e0405445558ba9203233d7f977cab07
Reviewed-on: https://review.whamcloud.com/33173
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>