Fix up r316081 by using nitems(cam_errbuf) instead of sizeof(cam_errbuf)

Fix up r316081 by using nitems(cam_errbuf) instead of sizeof(cam_errbuf)

Part of my original reasoning as far as converting the snprintf
calls was to permit switching over from char[] to wchar_t[] in the
future, as well as futureproof in case cam_errbuf's size was ever
changed.

Unfortunately, my approach was bugged because it conflated the
number of items with the size of the buffer, instead of the number of
elements being a fixed size != 1 byte.

Use nitems(..) instead which counts the quantity of items of a specific
type, as opposed to an unqualified sizeof(..) (which assumes that the
number of characters is equal to the buffer size).

MFC after:	2 months
Noted by:	cem
Sponsored by:	Dell EMC Isilon