octeontx2-pf: fix a buffer overflow in otx2_set_rxfh_context()

commit f927ef17c610e4ad1bc7cd86e252d529fecc135d from
git@git.assembla.com:cavium/WindRiver.linux.git

This function is called from ethtool_set_rxfh() and "*rss_context"
comes from the user.  Add some bounds checking to prevent memory
corruption.

Change-Id: Ica11b505d79e59f10a5ac5fb48ddccbeb24502ff
Fixes: 81a43620

 ("octeontx2-pf: Add RSS multi group support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/56145
Tested-by: sa_ip-sw-jenkins <sa_ip-sw-jenkins@marvell.com>
Reviewed-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>