Linux: Clear msghdr structure before use

Linux: Clear msghdr structure before use

The msghdr structure is used as a parameter to the Linux kernel
functions kernel_sendmsg() and kernel_recvmsg(). Some required fields
need to be set prior to calling these functions, but there are also
additional structure members that may not be used by the calling code.
Some of these fields may be initialized by the Linux kernel functions
being used, but there may be some that are left uninitialized.

To ensure that all fields in the msghdr structure are cleared, use
memset to zero the entire structure. This will eliminate the need to set
individual fields to 0 or NULL.

Change-Id: I7cdc7539084d13efedbb20e7ebd5c242693a628c
Reviewed-on: https://gerrit.openafs.org/15409
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>