rx: Save errno in pthread rxi_Sendmsg

rx: Save errno in pthread rxi_Sendmsg

Currently, our pthread version of rxi_Sendmsg uses 'errno' in some
logic if sendmsg fails, but we do so after calling functions that
might alter errno (e.g. fflush).

To make sure we get the correct errno value, save the value of errno
right after sendmsg returns an error. Reorganize this function a bit
to help make the logic easier to follow.

Change-Id: I6bf284bd75edb5404bb6771bb99a9381b0f8654d
Reviewed-on: https://gerrit.openafs.org/14423
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>