LU-14392 gnilnd: re-enable large I/o buffers

LU-14392 gnilnd: re-enable large I/o buffers

DVS on gni breaks the LNet 1M handshake of LNET_MAX_IOV.

Introduce GNILND_MAX_IOV with a 4M i/o maximum and a hint
LNET_MD_GNILND so LNet can accept the large buffer w/o complaint.

Test-Parameters: trivial
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I4e78c0022fdece0d6945bbcc47e2e64d4d181dca
Reviewed-on: https://review.whamcloud.com/41373
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>