tipc: Fix backport of b77413446408fdd256599daf00d5be72b5f3e7c6

Clang warns:

net/tipc/link.c:896:23: warning: variable 'hdr' is uninitialized when
used here [-Wuninitialized]
        imp = msg_importance(hdr);
                             ^~~
net/tipc/link.c:890:22: note: initialize the variable 'hdr' to silence
this warning
        struct tipc_msg *hdr;
                            ^
                             = NULL
1 warning generated.

The backport of commit b77413446408 ("tipc: fix NULL deref in
tipc_link_xmit()") to 4.9 as commit 310014f5 ("tipc: fix NULL deref
in tipc_link_xmit()") added the hdr initialization above the

    if (unlikely(msg_size(hdr) > mtu)) {

like in the upstream commit; however, in 4.9, that check is below imp's
first use because commit 365ad353c256 ("tipc: reduce risk of user
starvation during link congestion") is not present. This results in hdr
being used uninitialized.

Fix this by moving hdr's initialization before imp and after the if
check like the original backport did.

Cc: Hoang Le <hoang.h.le@dektech.com.au>
Cc: Jon Maloy <jon.maloy@ericsson.com>
Cc: Ying Xue <ying.xue@windriver.com>
Fixes: 310014f5 ("tipc: fix NULL deref in tipc_link_xmit()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>