EX-7601 tgt: objcount in RPC must be 1

EX-7601 tgt: objcount in RPC must be 1

Much of the BRW write code assumes objcount is one, but
there is some provision for multiple objects.

Since the code will break if we send it multiple objects,
add errors to make sure anyone changing it will notice.

This isn't strictly compression related, but compression
adds even more code which assumes this, so this protection
will be useful.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Idcbf33fd14d4b1bd179c9516bed07cca907008bc
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52990
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>