nfp: validate rtsym accesses fall within the symbol

With the accesses to rtsyms now all going via special helpers
we can easily make sure the driver is not reading past the
end of the symbol.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Francois H. Theron <francois.theron@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>