Fix va_list handling

• Add missing va_end's after corresponding va_start's to cleanup state
• Eliminate questionable bzero'ing of va_list passed in to do_buff_decode(..) and do_encode(..) from buff_{de,en}code_visit(..) and csio_{de,en}code_visit(..). Make va_list a pointer instead and pass NULL into the underlying functions to handler this in a portable way.
• Do some minor style(9) clean up in affected functions.

Differential Revision: https://reviews.freebsd.org/D6072
MFC after: 3 days
Reported by: cppcheck, Coverity
CID: 1018500-1018503
Reviewed by: cem
Sponsored by: EMC / Isilon Storage Division