LU-12981 lnet: Check MTU accurately

The existing check for MTU lacks checking for the KIOV/IOV cases,
and false positive triggered for very large incomming
buffer.

Cray-bug-id: LUS-7948
Signed-off-by: Alexey Lyashkov <c17817@cray.com>
Change-Id: Id3497e5f63470c24b2e51703fc564b02c9516aa6
Reviewed-on: https://review.whamcloud.com/36796
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>