LU-16211 o2iblnd: Avoid NULL md deref

LU-16211 o2iblnd: Avoid NULL md deref

struct lnet_msg::msg_md is NULL when a router is forwarding a
REPLY. ko2iblnd attempts to access this pointer on the receive path.
This causes a panic.

Test-Parameters: trivial
Fixes: 959304eac7 ("LU-15189 lnet: fix memory mapping.")
HPE-bug-id: LUS-11269
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: I0c1dbb1e0bcd3c17b278f358755d465f7bbbb2b0
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48777
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>